Merge pull request #261939 from tweag/check-by-name-intermediate

`nixpkgs-check-by-name`: Intermediate error representation refactor

10 files changed, 687 insertions, 350 deletions

diff --git a/pkgs/test/nixpkgs-check-by-name/Cargo.lock b/pkgs/test/nixpkgs-check-by-name/Cargo.lock
index aa4459c7cff..fc3aeb9fd79 100644
--- a/pkgs/test/nixpkgs-check-by-name/Cargo.lock
+++ b/pkgs/test/nixpkgs-check-by-name/Cargo.lock
@@ -163,6 +163,12 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7704b5fdd17b18ae31c4c1da5a2e0305a2bf17b5249300a9ee9ed7b72114c636"
 
 [[package]]
+name = "either"
+version = "1.9.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a26ae43d7bcc3b814de94796a5e736d4029efb0ee900c12e2d54c993ad1a1e07"
+
+[[package]]
 name = "errno"
 version = "0.3.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -219,6 +225,15 @@ dependencies = [
 ]
 
 [[package]]
+name = "itertools"
+version = "0.11.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b1c173a5686ce8bfa551b3563d0c2170bf24ca44da99c7ca4bfdab5418c3fe57"
+dependencies = [
+ "either",
+]
+
+[[package]]
 name = "itoa"
 version = "1.0.9"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -274,6 +289,7 @@ dependencies = [
  "anyhow",
  "clap",
  "colored",
+ "itertools",
  "lazy_static",
  "regex",
  "rnix",
diff --git a/pkgs/test/nixpkgs-check-by-name/Cargo.toml b/pkgs/test/nixpkgs-check-by-name/Cargo.toml
index 70b44d04820..1e6eaa1106d 100644
--- a/pkgs/test/nixpkgs-check-by-name/Cargo.toml
+++ b/pkgs/test/nixpkgs-check-by-name/Cargo.toml
@@ -13,6 +13,7 @@ serde = { version = "1.0.185", features = ["derive"] }
 anyhow = "1.0"
 lazy_static = "1.4.0"
 colored = "2.0.4"
+itertools = "0.11.0"
 
 [dev-dependencies]
 temp-env = "0.3.5"
diff --git a/pkgs/test/nixpkgs-check-by-name/README.md b/pkgs/test/nixpkgs-check-by-name/README.md
index 2d2db9a58bc..146cea0a64b 100644
--- a/pkgs/test/nixpkgs-check-by-name/README.md
+++ b/pkgs/test/nixpkgs-check-by-name/README.md
@@ -1,6 +1,6 @@
 # Nixpkgs pkgs/by-name checker
 
-This directory implements a program to check the [validity](#validity-checks) of the `pkgs/by-name` Nixpkgs directory once introduced.
+This directory implements a program to check the [validity](#validity-checks) of the `pkgs/by-name` Nixpkgs directory.
 It is being used by [this GitHub Actions workflow](../../../.github/workflows/check-by-name.yml).
 This is part of the implementation of [RFC 140](https://github.com/NixOS/rfcs/pull/140).
 
@@ -24,7 +24,7 @@ This API may be changed over time if the CI workflow making use of it is adjuste
   - `2`: If an unexpected I/O error occurs
 - Standard error:
   - Informative messages
-  - Error messages if validation is not successful
+  - Detected problems if validation is not successful
 
 ## Validity checks
 
diff --git a/pkgs/test/nixpkgs-check-by-name/src/eval.rs b/pkgs/test/nixpkgs-check-by-name/src/eval.rs
index 26836501c97..e4f986748b4 100644
--- a/pkgs/test/nixpkgs-check-by-name/src/eval.rs
+++ b/pkgs/test/nixpkgs-check-by-name/src/eval.rs
@@ -1,12 +1,12 @@
+use crate::nixpkgs_problem::NixpkgsProblem;
 use crate::structure;
-use crate::utils::ErrorWriter;
+use crate::validation::{self, Validation::Success};
 use crate::Version;
 use std::path::Path;
 
 use anyhow::Context;
 use serde::Deserialize;
 use std::collections::HashMap;
-use std::io;
 use std::path::PathBuf;
 use std::process;
 use tempfile::NamedTempFile;
@@ -40,12 +40,12 @@ const EXPR: &str = include_str!("eval.nix");
 /// Check that the Nixpkgs attribute values corresponding to the packages in pkgs/by-name are
 /// of the form `callPackage <package_file> { ... }`.
 /// See the `eval.nix` file for how this is achieved on the Nix side
-pub fn check_values<W: io::Write>(
+pub fn check_values(
     version: Version,
-    error_writer: &mut ErrorWriter<W>,
-    nixpkgs: &structure::Nixpkgs,
+    nixpkgs_path: &Path,
+    package_names: Vec<String>,
     eval_accessible_paths: Vec<&Path>,
-) -> anyhow::Result<()> {
+) -> validation::Result<()> {
     // Write the list of packages we need to check into a temporary JSON file.
     // This can then get read by the Nix evaluation.
     let attrs_file = NamedTempFile::new().context("Failed to create a temporary file")?;
@@ -55,7 +55,7 @@ pub fn check_values<W: io::Write>(
     // entry is needed.
     let attrs_file_path = attrs_file.path().canonicalize()?;
 
-    serde_json::to_writer(&attrs_file, &nixpkgs.package_names).context(format!(
+    serde_json::to_writer(&attrs_file, &package_names).context(format!(
         "Failed to serialise the package names to the temporary path {}",
         attrs_file_path.display()
     ))?;
@@ -87,9 +87,9 @@ pub fn check_values<W: io::Write>(
         .arg(&attrs_file_path)
         // Same for the nixpkgs to test
         .args(["--arg", "nixpkgsPath"])
-        .arg(&nixpkgs.path)
+        .arg(nixpkgs_path)
         .arg("-I")
-        .arg(&nixpkgs.path);
+        .arg(nixpkgs_path);
 
     // Also add extra paths that need to be accessible
     for path in eval_accessible_paths {
@@ -111,52 +111,54 @@ pub fn check_values<W: io::Write>(
             String::from_utf8_lossy(&result.stdout)
         ))?;
 
-    for package_name in &nixpkgs.package_names {
-        let relative_package_file = structure::Nixpkgs::relative_file_for_package(package_name);
-        let absolute_package_file = nixpkgs.path.join(&relative_package_file);
+    Ok(validation::sequence_(package_names.iter().map(
+        |package_name| {
+            let relative_package_file = structure::relative_file_for_package(package_name);
+            let absolute_package_file = nixpkgs_path.join(&relative_package_file);
 
-        if let Some(attribute_info) = actual_files.get(package_name) {
-            let valid = match &attribute_info.variant {
-                AttributeVariant::AutoCalled => true,
-                AttributeVariant::CallPackage { path, empty_arg } => {
-                    let correct_file = if let Some(call_package_path) = path {
-                        absolute_package_file == *call_package_path
-                    } else {
-                        false
-                    };
-                    // Only check for the argument to be non-empty if the version is V1 or
-                    // higher
-                    let non_empty = if version >= Version::V1 {
-                        !empty_arg
-                    } else {
-                        true
-                    };
-                    correct_file && non_empty
-                }
-                AttributeVariant::Other => false,
-            };
-
-            if !valid {
-                error_writer.write(&format!(
-                    "pkgs.{package_name}: This attribute is manually defined (most likely in pkgs/top-level/all-packages.nix), which is only allowed if the definition is of the form `pkgs.callPackage {} {{ ... }}` with a non-empty second argument.",
-                    relative_package_file.display()
-                ))?;
-                continue;
-            }
+            if let Some(attribute_info) = actual_files.get(package_name) {
+                let valid = match &attribute_info.variant {
+                    AttributeVariant::AutoCalled => true,
+                    AttributeVariant::CallPackage { path, empty_arg } => {
+                        let correct_file = if let Some(call_package_path) = path {
+                            absolute_package_file == *call_package_path
+                        } else {
+                            false
+                        };
+                        // Only check for the argument to be non-empty if the version is V1 or
+                        // higher
+                        let non_empty = if version >= Version::V1 {
+                            !empty_arg
+                        } else {
+                            true
+                        };
+                        correct_file && non_empty
+                    }
+                    AttributeVariant::Other => false,
+                };
 
-            if !attribute_info.is_derivation {
-                error_writer.write(&format!(
-                    "pkgs.{package_name}: This attribute defined by {} is not a derivation",
-                    relative_package_file.display()
-                ))?;
+                if !valid {
+                    NixpkgsProblem::WrongCallPackage {
+                        relative_package_file: relative_package_file.clone(),
+                        package_name: package_name.clone(),
+                    }
+                    .into()
+                } else if !attribute_info.is_derivation {
+                    NixpkgsProblem::NonDerivation {
+                        relative_package_file: relative_package_file.clone(),
+                        package_name: package_name.clone(),
+                    }
+                    .into()
+                } else {
+                    Success(())
+                }
+            } else {
+                NixpkgsProblem::UndefinedAttr {
+                    relative_package_file: relative_package_file.clone(),
+                    package_name: package_name.clone(),
+                }
+                .into()
             }
-        } else {
-            error_writer.write(&format!(
-                "pkgs.{package_name}: This attribute is not defined but it should be defined automatically as {}",
-                relative_package_file.display()
-            ))?;
-            continue;
-        }
-    }
-    Ok(())
+        },
+    )))
 }
diff --git a/pkgs/test/nixpkgs-check-by-name/src/main.rs b/pkgs/test/nixpkgs-check-by-name/src/main.rs
index 5d28077ae4a..4cabf8f446f 100644
--- a/pkgs/test/nixpkgs-check-by-name/src/main.rs
+++ b/pkgs/test/nixpkgs-check-by-name/src/main.rs
@@ -1,16 +1,19 @@
 mod eval;
+mod nixpkgs_problem;
 mod references;
 mod structure;
 mod utils;
+mod validation;
 
+use crate::structure::check_structure;
+use crate::validation::Validation::Failure;
+use crate::validation::Validation::Success;
 use anyhow::Context;
 use clap::{Parser, ValueEnum};
 use colored::Colorize;
 use std::io;
 use std::path::{Path, PathBuf};
 use std::process::ExitCode;
-use structure::Nixpkgs;
-use utils::ErrorWriter;
 
 /// Program to check the validity of pkgs/by-name
 #[derive(Parser, Debug)]
@@ -63,8 +66,8 @@ fn main() -> ExitCode {
 ///
 /// # Return value
 /// - `Err(e)` if an I/O-related error `e` occurred.
-/// - `Ok(false)` if the structure is invalid, all the structural errors have been written to `error_writer`.
-/// - `Ok(true)` if the structure is valid, nothing will have been written to `error_writer`.
+/// - `Ok(false)` if there are problems, all of which will be written to `error_writer`.
+/// - `Ok(true)` if there are no problems
 pub fn check_nixpkgs<W: io::Write>(
     nixpkgs_path: &Path,
     version: Version,
@@ -76,31 +79,38 @@ pub fn check_nixpkgs<W: io::Write>(
         nixpkgs_path.display()
     ))?;
 
-    // Wraps the error_writer to print everything in red, and tracks whether anything was printed
-    // at all. Later used to figure out if the structure was valid or not.
-    let mut error_writer = ErrorWriter::new(error_writer);
-
-    if !nixpkgs_path.join(structure::BASE_SUBPATH).exists() {
+    let check_result = if !nixpkgs_path.join(utils::BASE_SUBPATH).exists() {
         eprintln!(
             "Given Nixpkgs path does not contain a {} subdirectory, no check necessary.",
-            structure::BASE_SUBPATH
+            utils::BASE_SUBPATH
         );
+        Success(())
     } else {
-        let nixpkgs = Nixpkgs::new(&nixpkgs_path, &mut error_writer)?;
+        match check_structure(&nixpkgs_path)? {
+            Failure(errors) => Failure(errors),
+            Success(package_names) =>
+            // Only if we could successfully parse the structure, we do the evaluation checks
+            {
+                eval::check_values(version, &nixpkgs_path, package_names, eval_accessible_paths)?
+            }
+        }
+    };
 
-        if error_writer.empty {
-            // Only if we could successfully parse the structure, we do the semantic checks
-            eval::check_values(version, &mut error_writer, &nixpkgs, eval_accessible_paths)?;
-            references::check_references(&mut error_writer, &nixpkgs)?;
+    match check_result {
+        Failure(errors) => {
+            for error in errors {
+                writeln!(error_writer, "{}", error.to_string().red())?
+            }
+            Ok(false)
         }
+        Success(_) => Ok(true),
     }
-    Ok(error_writer.empty)
 }
 
 #[cfg(test)]
 mod tests {
     use crate::check_nixpkgs;
-    use crate::structure;
+    use crate::utils;
     use crate::Version;
     use anyhow::Context;
     use std::fs;
@@ -145,7 +155,7 @@ mod tests {
             return Ok(());
         }
 
-        let base = path.join(structure::BASE_SUBPATH);
+        let base = path.join(utils::BASE_SUBPATH);
 
         fs::create_dir_all(base.join("fo/foo"))?;
         fs::write(base.join("fo/foo/package.nix"), "{ someDrv }: someDrv")?;
diff --git a/pkgs/test/nixpkgs-check-by-name/src/nixpkgs_problem.rs b/pkgs/test/nixpkgs-check-by-name/src/nixpkgs_problem.rs
new file mode 100644
index 00000000000..2344a8cc132
--- /dev/null
+++ b/pkgs/test/nixpkgs-check-by-name/src/nixpkgs_problem.rs
@@ -0,0 +1,218 @@
+use crate::utils::PACKAGE_NIX_FILENAME;
+use rnix::parser::ParseError;
+use std::ffi::OsString;
+use std::fmt;
+use std::io;
+use std::path::PathBuf;
+
+/// Any problem that can occur when checking Nixpkgs
+pub enum NixpkgsProblem {
+    ShardNonDir {
+        relative_shard_path: PathBuf,
+    },
+    InvalidShardName {
+        relative_shard_path: PathBuf,
+        shard_name: String,
+    },
+    PackageNonDir {
+        relative_package_dir: PathBuf,
+    },
+    CaseSensitiveDuplicate {
+        relative_shard_path: PathBuf,
+        first: OsString,
+        second: OsString,
+    },
+    InvalidPackageName {
+        relative_package_dir: PathBuf,
+        package_name: String,
+    },
+    IncorrectShard {
+        relative_package_dir: PathBuf,
+        correct_relative_package_dir: PathBuf,
+    },
+    PackageNixNonExistent {
+        relative_package_dir: PathBuf,
+    },
+    PackageNixDir {
+        relative_package_dir: PathBuf,
+    },
+    UndefinedAttr {
+        relative_package_file: PathBuf,
+        package_name: String,
+    },
+    WrongCallPackage {
+        relative_package_file: PathBuf,
+        package_name: String,
+    },
+    NonDerivation {
+        relative_package_file: PathBuf,
+        package_name: String,
+    },
+    OutsideSymlink {
+        relative_package_dir: PathBuf,
+        subpath: PathBuf,
+    },
+    UnresolvableSymlink {
+        relative_package_dir: PathBuf,
+        subpath: PathBuf,
+        io_error: io::Error,
+    },
+    CouldNotParseNix {
+        relative_package_dir: PathBuf,
+        subpath: PathBuf,
+        error: ParseError,
+    },
+    PathInterpolation {
+        relative_package_dir: PathBuf,
+        subpath: PathBuf,
+        line: usize,
+        text: String,
+    },
+    SearchPath {
+        relative_package_dir: PathBuf,
+        subpath: PathBuf,
+        line: usize,
+        text: String,
+    },
+    OutsidePathReference {
+        relative_package_dir: PathBuf,
+        subpath: PathBuf,
+        line: usize,
+        text: String,
+    },
+    UnresolvablePathReference {
+        relative_package_dir: PathBuf,
+        subpath: PathBuf,
+        line: usize,
+        text: String,
+        io_error: io::Error,
+    },
+}
+
+impl fmt::Display for NixpkgsProblem {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self {
+            NixpkgsProblem::ShardNonDir { relative_shard_path } =>
+                write!(
+                    f,
+                    "{}: This is a file, but it should be a directory.",
+                    relative_shard_path.display(),
+                ),
+            NixpkgsProblem::InvalidShardName { relative_shard_path, shard_name } =>
+                write!(
+                    f,
+                    "{}: Invalid directory name \"{shard_name}\", must be at most 2 ASCII characters consisting of a-z, 0-9, \"-\" or \"_\".",
+                    relative_shard_path.display()
+                ),
+            NixpkgsProblem::PackageNonDir { relative_package_dir } =>
+                write!(
+                    f,
+                    "{}: This path is a file, but it should be a directory.",
+                    relative_package_dir.display(),
+                ),
+            NixpkgsProblem::CaseSensitiveDuplicate { relative_shard_path, first, second } =>
+                write!(
+                    f,
+                    "{}: Duplicate case-sensitive package directories {first:?} and {second:?}.",
+                    relative_shard_path.display(),
+                ),
+            NixpkgsProblem::InvalidPackageName { relative_package_dir, package_name } =>
+                write!(
+                    f,
+                    "{}: Invalid package directory name \"{package_name}\", must be ASCII characters consisting of a-z, A-Z, 0-9, \"-\" or \"_\".",
+                    relative_package_dir.display(),
+                ),
+            NixpkgsProblem::IncorrectShard { relative_package_dir, correct_relative_package_dir } =>
+                write!(
+                    f,
+                    "{}: Incorrect directory location, should be {} instead.",
+                    relative_package_dir.display(),
+                    correct_relative_package_dir.display(),
+                ),
+            NixpkgsProblem::PackageNixNonExistent { relative_package_dir } =>
+                write!(
+                    f,
+                    "{}: Missing required \"{PACKAGE_NIX_FILENAME}\" file.",
+                    relative_package_dir.display(),
+                ),
+            NixpkgsProblem::PackageNixDir { relative_package_dir } =>
+                write!(
+                    f,
+                    "{}: \"{PACKAGE_NIX_FILENAME}\" must be a file.",
+                    relative_package_dir.display(),
+                ),
+            NixpkgsProblem::UndefinedAttr { relative_package_file, package_name } =>
+                write!(
+                    f,
+                    "pkgs.{package_name}: This attribute is not defined but it should be defined automatically as {}",
+                    relative_package_file.display()
+                ),
+            NixpkgsProblem::WrongCallPackage { relative_package_file, package_name } =>
+                write!(
+                    f,
+                    "pkgs.{package_name}: This attribute is manually defined (most likely in pkgs/top-level/all-packages.nix), which is only allowed if the definition is of the form `pkgs.callPackage {} {{ ... }}` with a non-empty second argument.",
+                    relative_package_file.display()
+                ),
+            NixpkgsProblem::NonDerivation { relative_package_file, package_name } =>
+                write!(
+                    f,
+                    "pkgs.{package_name}: This attribute defined by {} is not a derivation",
+                    relative_package_file.display()
+                ),
+            NixpkgsProblem::OutsideSymlink { relative_package_dir, subpath } =>
+                write!(
+                    f,
+                    "{}: Path {} is a symlink pointing to a path outside the directory of that package.",
+                    relative_package_dir.display(),
+                    subpath.display(),
+                ),
+            NixpkgsProblem::UnresolvableSymlink { relative_package_dir, subpath, io_error } =>
+                write!(
+                    f,
+                    "{}: Path {} is a symlink which cannot be resolved: {io_error}.",
+                    relative_package_dir.display(),
+                    subpath.display(),
+                ),
+            NixpkgsProblem::CouldNotParseNix { relative_package_dir, subpath, error } =>
+                write!(
+                    f,
+                    "{}: File {} could not be parsed by rnix: {}",
+                    relative_package_dir.display(),
+                    subpath.display(),
+                    error,
+                ),
+            NixpkgsProblem::PathInterpolation { relative_package_dir, subpath, line, text } =>
+                write!(
+                    f,
+                    "{}: File {} at line {line} contains the path expression \"{}\", which is not yet supported and may point outside the directory of that package.",
+                    relative_package_dir.display(),
+                    subpath.display(),
+                    text
+                ),
+            NixpkgsProblem::SearchPath { relative_package_dir, subpath, line, text } =>
+                write!(
+                    f,
+                    "{}: File {} at line {line} contains the nix search path expression \"{}\" which may point outside the directory of that package.",
+                    relative_package_dir.display(),
+                    subpath.display(),
+                    text
+                ),
+            NixpkgsProblem::OutsidePathReference { relative_package_dir, subpath, line, text } =>
+                write!(
+                    f,
+                    "{}: File {} at line {line} contains the path expression \"{}\" which may point outside the directory of that package.",
+                    relative_package_dir.display(),
+                    subpath.display(),
+                    text,
+                ),
+            NixpkgsProblem::UnresolvablePathReference { relative_package_dir, subpath, line, text, io_error } =>
+                write!(
+                    f,
+                    "{}: File {} at line {line} contains the path expression \"{}\" which cannot be resolved: {io_error}.",
+                    relative_package_dir.display(),
+                    subpath.display(),
+                    text,
+                ),
+        }
+    }
+}
diff --git a/pkgs/test/nixpkgs-check-by-name/src/references.rs b/pkgs/test/nixpkgs-check-by-name/src/references.rs
index 16dc60729c4..0561a9b22e8 100644
--- a/pkgs/test/nixpkgs-check-by-name/src/references.rs
+++ b/pkgs/test/nixpkgs-check-by-name/src/references.rs
@@ -1,105 +1,98 @@
-use crate::structure::Nixpkgs;
+use crate::nixpkgs_problem::NixpkgsProblem;
 use crate::utils;
-use crate::utils::{ErrorWriter, LineIndex};
+use crate::utils::LineIndex;
+use crate::validation::{self, ResultIteratorExt, Validation::Success};
 
 use anyhow::Context;
 use rnix::{Root, SyntaxKind::NODE_PATH};
 use std::ffi::OsStr;
 use std::fs::read_to_string;
-use std::io;
-use std::path::{Path, PathBuf};
-
-/// Small helper so we don't need to pass in the same arguments to all functions
-struct PackageContext<'a, W: io::Write> {
-    error_writer: &'a mut ErrorWriter<W>,
-    /// The package directory relative to Nixpkgs, such as `pkgs/by-name/fo/foo`
-    relative_package_dir: &'a PathBuf,
-    /// The absolute package directory
-    absolute_package_dir: &'a PathBuf,
-}
+use std::path::Path;
 
 /// Check that every package directory in pkgs/by-name doesn't link to outside that directory.
 /// Both symlinks and Nix path expressions are checked.
-pub fn check_references<W: io::Write>(
-    error_writer: &mut ErrorWriter<W>,
-    nixpkgs: &Nixpkgs,
-) -> anyhow::Result<()> {
-    // Check the directories for each package separately
-    for package_name in &nixpkgs.package_names {
-        let relative_package_dir = Nixpkgs::relative_dir_for_package(package_name);
-        let mut context = PackageContext {
-            error_writer,
-            relative_package_dir: &relative_package_dir,
-            absolute_package_dir: &nixpkgs.path.join(&relative_package_dir),
-        };
-
-        // The empty argument here is the subpath under the package directory to check
-        // An empty one means the package directory itself
-        check_path(&mut context, Path::new("")).context(format!(
-            "While checking the references in package directory {}",
-            relative_package_dir.display()
-        ))?;
-    }
-    Ok(())
+pub fn check_references(
+    relative_package_dir: &Path,
+    absolute_package_dir: &Path,
+) -> validation::Result<()> {
+    // The empty argument here is the subpath under the package directory to check
+    // An empty one means the package directory itself
+    check_path(relative_package_dir, absolute_package_dir, Path::new("")).context(format!(
+        "While checking the references in package directory {}",
+        relative_package_dir.display()
+    ))
 }
 
 /// Checks for a specific path to not have references outside
-fn check_path<W: io::Write>(context: &mut PackageContext<W>, subpath: &Path) -> anyhow::Result<()> {
-    let path = context.absolute_package_dir.join(subpath);
+fn check_path(
+    relative_package_dir: &Path,
+    absolute_package_dir: &Path,
+    subpath: &Path,
+) -> validation::Result<()> {
+    let path = absolute_package_dir.join(subpath);
 
-    if path.is_symlink() {
+    Ok(if path.is_symlink() {
         // Check whether the symlink resolves to outside the package directory
         match path.canonicalize() {
             Ok(target) => {
                 // No need to handle the case of it being inside the directory, since we scan through the
                 // entire directory recursively anyways
-                if let Err(_prefix_error) = target.strip_prefix(context.absolute_package_dir) {
-                    context.error_writer.write(&format!(
-                        "{}: Path {} is a symlink pointing to a path outside the directory of that package.",
-                        context.relative_package_dir.display(),
-                        subpath.display(),
-                    ))?;
+                if let Err(_prefix_error) = target.strip_prefix(absolute_package_dir) {
+                    NixpkgsProblem::OutsideSymlink {
+                        relative_package_dir: relative_package_dir.to_path_buf(),
+                        subpath: subpath.to_path_buf(),
+                    }
+                    .into()
+                } else {
+                    Success(())
                 }
             }
-            Err(e) => {
-                context.error_writer.write(&format!(
-                    "{}: Path {} is a symlink which cannot be resolved: {e}.",
-                    context.relative_package_dir.display(),
-                    subpath.display(),
-                ))?;
+            Err(io_error) => NixpkgsProblem::UnresolvableSymlink {
+                relative_package_dir: relative_package_dir.to_path_buf(),
+                subpath: subpath.to_path_buf(),
+                io_error,
             }
+            .into(),
         }
     } else if path.is_dir() {
         // Recursively check each entry
-        for entry in utils::read_dir_sorted(&path)? {
-            let entry_subpath = subpath.join(entry.file_name());
-            check_path(context, &entry_subpath)
-                .context(format!("Error while recursing into {}", subpath.display()))?
-        }
+        validation::sequence_(
+            utils::read_dir_sorted(&path)?
+                .into_iter()
+                .map(|entry| {
+                    let entry_subpath = subpath.join(entry.file_name());
+                    check_path(relative_package_dir, absolute_package_dir, &entry_subpath)
+                        .context(format!("Error while recursing into {}", subpath.display()))
+                })
+                .collect_vec()?,
+        )
     } else if path.is_file() {
         // Only check Nix files
         if let Some(ext) = path.extension() {
             if ext == OsStr::new("nix") {
-                check_nix_file(context, subpath).context(format!(
-                    "Error while checking Nix file {}",
-                    subpath.display()
-                ))?
+                check_nix_file(relative_package_dir, absolute_package_dir, subpath).context(
+                    format!("Error while checking Nix file {}", subpath.display()),
+                )?
+            } else {
+                Success(())
             }
+        } else {
+            Success(())
         }
     } else {
         // This should never happen, git doesn't support other file types
         anyhow::bail!("Unsupported file type for path {}", subpath.display());
-    }
-    Ok(())
+    })
 }
 
 /// Check whether a nix file contains path expression references pointing outside the package
 /// directory
-fn check_nix_file<W: io::Write>(
-    context: &mut PackageContext<W>,
+fn check_nix_file(
+    relative_package_dir: &Path,
+    absolute_package_dir: &Path,
     subpath: &Path,
-) -> anyhow::Result<()> {
-    let path = context.absolute_package_dir.join(subpath);
+) -> validation::Result<()> {
+    let path = absolute_package_dir.join(subpath);
     let parent_dir = path.parent().context(format!(
         "Could not get parent of path {}",
         subpath.display()
@@ -110,75 +103,73 @@ fn check_nix_file<W: io::Write>(
 
     let root = Root::parse(&contents);
     if let Some(error) = root.errors().first() {
-        context.error_writer.write(&format!(
-            "{}: File {} could not be parsed by rnix: {}",
-            context.relative_package_dir.display(),
-            subpath.display(),
-            error,
-        ))?;
-        return Ok(());
+        return Ok(NixpkgsProblem::CouldNotParseNix {
+            relative_package_dir: relative_package_dir.to_path_buf(),
+            subpath: subpath.to_path_buf(),
+            error: error.clone(),
+        }
+        .into());
     }
 
     let line_index = LineIndex::new(&contents);
 
-    for node in root.syntax().descendants() {
-        // We're only interested in Path expressions
-        if node.kind() != NODE_PATH {
-            continue;
-        }
-
-        let text = node.text().to_string();
-        let line = line_index.line(node.text_range().start().into());
-
-        // Filters out ./foo/${bar}/baz
-        // TODO: We can just check ./foo
-        if node.children().count() != 0 {
-            context.error_writer.write(&format!(
-                "{}: File {} at line {line} contains the path expression \"{}\", which is not yet supported and may point outside the directory of that package.",
-                context.relative_package_dir.display(),
-                subpath.display(),
-                text
-            ))?;
-            continue;
-        }
-
-        // Filters out search paths like <nixpkgs>
-        if text.starts_with('<') {
-            context.error_writer.write(&format!(
-                "{}: File {} at line {line} contains the nix search path expression \"{}\" which may point outside the directory of that package.",
-                context.relative_package_dir.display(),
-                subpath.display(),
-                text
-            ))?;
-            continue;
-        }
-
-        // Resolves the reference of the Nix path
-        // turning `../baz` inside `/foo/bar/default.nix` to `/foo/baz`
-        match parent_dir.join(Path::new(&text)).canonicalize() {
-            Ok(target) => {
-                // Then checking if it's still in the package directory
-                // No need to handle the case of it being inside the directory, since we scan through the
-                // entire directory recursively anyways
-                if let Err(_prefix_error) = target.strip_prefix(context.absolute_package_dir) {
-                    context.error_writer.write(&format!(
-                        "{}: File {} at line {line} contains the path expression \"{}\" which may point outside the directory of that package.",
-                        context.relative_package_dir.display(),
-                        subpath.display(),
-                        text,
-                    ))?;
+    Ok(validation::sequence_(root.syntax().descendants().map(
+        |node| {
+            let text = node.text().to_string();
+            let line = line_index.line(node.text_range().start().into());
+
+            if node.kind() != NODE_PATH {
+                // We're only interested in Path expressions
+                Success(())
+            } else if node.children().count() != 0 {
+                // Filters out ./foo/${bar}/baz
+                // TODO: We can just check ./foo
+                NixpkgsProblem::PathInterpolation {
+                    relative_package_dir: relative_package_dir.to_path_buf(),
+                    subpath: subpath.to_path_buf(),
+                    line,
+                    text,
                 }
-            }
-            Err(e) => {
-                context.error_writer.write(&format!(
-                    "{}: File {} at line {line} contains the path expression \"{}\" which cannot be resolved: {e}.",
-                    context.relative_package_dir.display(),
-                    subpath.display(),
+                .into()
+            } else if text.starts_with('<') {
+                // Filters out search paths like <nixpkgs>
+                NixpkgsProblem::SearchPath {
+                    relative_package_dir: relative_package_dir.to_path_buf(),
+                    subpath: subpath.to_path_buf(),
+                    line,
                     text,
-                ))?;
+                }
+                .into()
+            } else {
+                // Resolves the reference of the Nix path
+                // turning `../baz` inside `/foo/bar/default.nix` to `/foo/baz`
+                match parent_dir.join(Path::new(&text)).canonicalize() {
+                    Ok(target) => {
+                        // Then checking if it's still in the package directory
+                        // No need to handle the case of it being inside the directory, since we scan through the
+                        // entire directory recursively anyways
+                        if let Err(_prefix_error) = target.strip_prefix(absolute_package_dir) {
+                            NixpkgsProblem::OutsidePathReference {
+                                relative_package_dir: relative_package_dir.to_path_buf(),
+                                subpath: subpath.to_path_buf(),
+                                line,
+                                text,
+                            }
+                            .into()
+                        } else {
+                            Success(())
+                        }
+                    }
+                    Err(e) => NixpkgsProblem::UnresolvablePathReference {
+                        relative_package_dir: relative_package_dir.to_path_buf(),
+                        subpath: subpath.to_path_buf(),
+                        line,
+                        text,
+                        io_error: e,
+                    }
+                    .into(),
+                }
             }
-        };
-    }
-
-    Ok(())
+        },
+    )))
 }
diff --git a/pkgs/test/nixpkgs-check-by-name/src/structure.rs b/pkgs/test/nixpkgs-check-by-name/src/structure.rs
index ea80128e487..4051ca037c9 100644
--- a/pkgs/test/nixpkgs-check-by-name/src/structure.rs
+++ b/pkgs/test/nixpkgs-check-by-name/src/structure.rs
@@ -1,152 +1,170 @@
+use crate::nixpkgs_problem::NixpkgsProblem;
+use crate::references;
 use crate::utils;
-use crate::utils::ErrorWriter;
+use crate::utils::{BASE_SUBPATH, PACKAGE_NIX_FILENAME};
+use crate::validation::{self, ResultIteratorExt, Validation::Success};
+use itertools::concat;
 use lazy_static::lazy_static;
 use regex::Regex;
-use std::collections::HashMap;
-use std::io;
+use std::fs::DirEntry;
 use std::path::{Path, PathBuf};
 
-pub const BASE_SUBPATH: &str = "pkgs/by-name";
-pub const PACKAGE_NIX_FILENAME: &str = "package.nix";
-
 lazy_static! {
     static ref SHARD_NAME_REGEX: Regex = Regex::new(r"^[a-z0-9_-]{1,2}$").unwrap();
     static ref PACKAGE_NAME_REGEX: Regex = Regex::new(r"^[a-zA-Z0-9_-]+$").unwrap();
 }
 
-/// Contains information about the structure of the pkgs/by-name directory of a Nixpkgs
-pub struct Nixpkgs {
-    /// The path to nixpkgs
-    pub path: PathBuf,
-    /// The names of all packages declared in pkgs/by-name
-    pub package_names: Vec<String>,
-}
-
-impl Nixpkgs {
-    // Some utility functions for the basic structure
+// Some utility functions for the basic structure
 
-    pub fn shard_for_package(package_name: &str) -> String {
-        package_name.to_lowercase().chars().take(2).collect()
-    }
-
-    pub fn relative_dir_for_shard(shard_name: &str) -> PathBuf {
-        PathBuf::from(format!("{BASE_SUBPATH}/{shard_name}"))
-    }
+pub fn shard_for_package(package_name: &str) -> String {
+    package_name.to_lowercase().chars().take(2).collect()
+}
 
-    pub fn relative_dir_for_package(package_name: &str) -> PathBuf {
-        Nixpkgs::relative_dir_for_shard(&Nixpkgs::shard_for_package(package_name))
-            .join(package_name)
-    }
+pub fn relative_dir_for_shard(shard_name: &str) -> PathBuf {
+    PathBuf::from(format!("{BASE_SUBPATH}/{shard_name}"))
+}
 
-    pub fn relative_file_for_package(package_name: &str) -> PathBuf {
-        Nixpkgs::relative_dir_for_package(package_name).join(PACKAGE_NIX_FILENAME)
-    }
+pub fn relative_dir_for_package(package_name: &str) -> PathBuf {
+    relative_dir_for_shard(&shard_for_package(package_name)).join(package_name)
 }
 
-impl Nixpkgs {
-    /// Read the structure of a Nixpkgs directory, displaying errors on the writer.
-    /// May return early with I/O errors.
-    pub fn new<W: io::Write>(
-        path: &Path,
-        error_writer: &mut ErrorWriter<W>,
-    ) -> anyhow::Result<Nixpkgs> {
-        let base_dir = path.join(BASE_SUBPATH);
+pub fn relative_file_for_package(package_name: &str) -> PathBuf {
+    relative_dir_for_package(package_name).join(PACKAGE_NIX_FILENAME)
+}
 
-        let mut package_names = Vec::new();
+/// Check the structure of Nixpkgs, returning the attribute names that are defined in
+/// `pkgs/by-name`
+pub fn check_structure(path: &Path) -> validation::Result<Vec<String>> {
+    let base_dir = path.join(BASE_SUBPATH);
 
-        for shard_entry in utils::read_dir_sorted(&base_dir)? {
+    let shard_results = utils::read_dir_sorted(&base_dir)?
+        .into_iter()
+        .map(|shard_entry| -> validation::Result<_> {
             let shard_path = shard_entry.path();
             let shard_name = shard_entry.file_name().to_string_lossy().into_owned();
-            let relative_shard_path = Nixpkgs::relative_dir_for_shard(&shard_name);
+            let relative_shard_path = relative_dir_for_shard(&shard_name);
 
-            if shard_name == "README.md" {
+            Ok(if shard_name == "README.md" {
                 // README.md is allowed to be a file and not checked
-                continue;
-            }
-
-            if !shard_path.is_dir() {
-                error_writer.write(&format!(
-                    "{}: This is a file, but it should be a directory.",
-                    relative_shard_path.display(),
-                ))?;
-                // we can't check for any other errors if it's a file, since there's no subdirectories to check
-                continue;
-            }
-
-            let shard_name_valid = SHARD_NAME_REGEX.is_match(&shard_name);
-            if !shard_name_valid {
-                error_writer.write(&format!(
-                    "{}: Invalid directory name \"{shard_name}\", must be at most 2 ASCII characters consisting of a-z, 0-9, \"-\" or \"_\".",
-                    relative_shard_path.display()
-                ))?;
-            }
-
-            let mut unique_package_names = HashMap::new();
-
-            for package_entry in utils::read_dir_sorted(&shard_path)? {
-                let package_path = package_entry.path();
-                let package_name = package_entry.file_name().to_string_lossy().into_owned();
-                let relative_package_dir =
-                    PathBuf::from(format!("{BASE_SUBPATH}/{shard_name}/{package_name}"));
-
-                if !package_path.is_dir() {
-                    error_writer.write(&format!(
-                        "{}: This path is a file, but it should be a directory.",
-                        relative_package_dir.display(),
-                    ))?;
-                    continue;
-                }
 
-                if let Some(duplicate_package_name) =
-                    unique_package_names.insert(package_name.to_lowercase(), package_name.clone())
-                {
-                    error_writer.write(&format!(
-                        "{}: Duplicate case-sensitive package directories \"{duplicate_package_name}\" and \"{package_name}\".",
-                        relative_shard_path.display(),
-                    ))?;
+                Success(vec![])
+            } else if !shard_path.is_dir() {
+                NixpkgsProblem::ShardNonDir {
+                    relative_shard_path: relative_shard_path.clone(),
                 }
-
-                let package_name_valid = PACKAGE_NAME_REGEX.is_match(&package_name);
-                if !package_name_valid {
-                    error_writer.write(&format!(
-                        "{}: Invalid package directory name \"{package_name}\", must be ASCII characters consisting of a-z, A-Z, 0-9, \"-\" or \"_\".",
-                        relative_package_dir.display(),
-                    ))?;
-                }
-
-                let correct_relative_package_dir = Nixpkgs::relative_dir_for_package(&package_name);
-                if relative_package_dir != correct_relative_package_dir {
-                    // Only show this error if we have a valid shard and package name
-                    // Because if one of those is wrong, you should fix that first
-                    if shard_name_valid && package_name_valid {
-                        error_writer.write(&format!(
-                            "{}: Incorrect directory location, should be {} instead.",
-                            relative_package_dir.display(),
-                            correct_relative_package_dir.display(),
-                        ))?;
+                .into()
+                // we can't check for any other errors if it's a file, since there's no subdirectories to check
+            } else {
+                let shard_name_valid = SHARD_NAME_REGEX.is_match(&shard_name);
+                let result = if !shard_name_valid {
+                    NixpkgsProblem::InvalidShardName {
+                        relative_shard_path: relative_shard_path.clone(),
+                        shard_name: shard_name.clone(),
                     }
-                }
+                    .into()
+                } else {
+                    Success(())
+                };
+
+                let entries = utils::read_dir_sorted(&shard_path)?;
+
+                let duplicate_results = entries
+                    .iter()
+                    .zip(entries.iter().skip(1))
+                    .filter(|(l, r)| {
+                        l.file_name().to_ascii_lowercase() == r.file_name().to_ascii_lowercase()
+                    })
+                    .map(|(l, r)| {
+                        NixpkgsProblem::CaseSensitiveDuplicate {
+                            relative_shard_path: relative_shard_path.clone(),
+                            first: l.file_name(),
+                            second: r.file_name(),
+                        }
+                        .into()
+                    });
+
+                let result = result.and(validation::sequence_(duplicate_results));
+
+                let package_results = entries
+                    .into_iter()
+                    .map(|package_entry| {
+                        check_package(path, &shard_name, shard_name_valid, package_entry)
+                    })
+                    .collect_vec()?;
+
+                result.and(validation::sequence(package_results))
+            })
+        })
+        .collect_vec()?;
 
-                let package_nix_path = package_path.join(PACKAGE_NIX_FILENAME);
-                if !package_nix_path.exists() {
-                    error_writer.write(&format!(
-                        "{}: Missing required \"{PACKAGE_NIX_FILENAME}\" file.",
-                        relative_package_dir.display(),
-                    ))?;
-                } else if package_nix_path.is_dir() {
-                    error_writer.write(&format!(
-                        "{}: \"{PACKAGE_NIX_FILENAME}\" must be a file.",
-                        relative_package_dir.display(),
-                    ))?;
-                }
+    // Combine the package names conatained within each shard into a longer list
+    Ok(validation::sequence(shard_results).map(concat))
+}
 
-                package_names.push(package_name.clone());
-            }
+fn check_package(
+    path: &Path,
+    shard_name: &str,
+    shard_name_valid: bool,
+    package_entry: DirEntry,
+) -> validation::Result<String> {
+    let package_path = package_entry.path();
+    let package_name = package_entry.file_name().to_string_lossy().into_owned();
+    let relative_package_dir = PathBuf::from(format!("{BASE_SUBPATH}/{shard_name}/{package_name}"));
+
+    Ok(if !package_path.is_dir() {
+        NixpkgsProblem::PackageNonDir {
+            relative_package_dir: relative_package_dir.clone(),
         }
-
-        Ok(Nixpkgs {
-            path: path.to_owned(),
-            package_names,
-        })
-    }
+        .into()
+    } else {
+        let package_name_valid = PACKAGE_NAME_REGEX.is_match(&package_name);
+        let result = if !package_name_valid {
+            NixpkgsProblem::InvalidPackageName {
+                relative_package_dir: relative_package_dir.clone(),
+                package_name: package_name.clone(),
+            }
+            .into()
+        } else {
+            Success(())
+        };
+
+        let correct_relative_package_dir = relative_dir_for_package(&package_name);
+        let result = result.and(if relative_package_dir != correct_relative_package_dir {
+            // Only show this error if we have a valid shard and package name
+            // Because if one of those is wrong, you should fix that first
+            if shard_name_valid && package_name_valid {
+                NixpkgsProblem::IncorrectShard {
+                    relative_package_dir: relative_package_dir.clone(),
+                    correct_relative_package_dir: correct_relative_package_dir.clone(),
+                }
+                .into()
+            } else {
+                Success(())
+            }
+        } else {
+            Success(())
+        });
+
+        let package_nix_path = package_path.join(PACKAGE_NIX_FILENAME);
+        let result = result.and(if !package_nix_path.exists() {
+            NixpkgsProblem::PackageNixNonExistent {
+                relative_package_dir: relative_package_dir.clone(),
+            }
+            .into()
+        } else if package_nix_path.is_dir() {
+            NixpkgsProblem::PackageNixDir {
+                relative_package_dir: relative_package_dir.clone(),
+            }
+            .into()
+        } else {
+            Success(())
+        });
+
+        let result = result.and(references::check_references(
+            &relative_package_dir,
+            &path.join(&relative_package_dir),
+        )?);
+
+        result.map(|_| package_name.clone())
+    })
 }
diff --git a/pkgs/test/nixpkgs-check-by-name/src/utils.rs b/pkgs/test/nixpkgs-check-by-name/src/utils.rs
index 325c736eca9..5cc4a0863ba 100644
--- a/pkgs/test/nixpkgs-check-by-name/src/utils.rs
+++ b/pkgs/test/nixpkgs-check-by-name/src/utils.rs
@@ -1,9 +1,11 @@
 use anyhow::Context;
-use colored::Colorize;
 use std::fs;
 use std::io;
 use std::path::Path;
 
+pub const BASE_SUBPATH: &str = "pkgs/by-name";
+pub const PACKAGE_NIX_FILENAME: &str = "package.nix";
+
 /// Deterministic file listing so that tests are reproducible
 pub fn read_dir_sorted(base_dir: &Path) -> anyhow::Result<Vec<fs::DirEntry>> {
     let listing = base_dir
@@ -47,26 +49,3 @@ impl LineIndex {
         }
     }
 }
-
-/// A small wrapper around a generic io::Write specifically for errors:
-/// - Print everything in red to signal it's an error
-/// - Keep track of whether anything was printed at all, so that
-///   it can be queried whether any errors were encountered at all
-pub struct ErrorWriter<W> {
-    pub writer: W,
-    pub empty: bool,
-}
-
-impl<W: io::Write> ErrorWriter<W> {
-    pub fn new(writer: W) -> ErrorWriter<W> {
-        ErrorWriter {
-            writer,
-            empty: true,
-        }
-    }
-
-    pub fn write(&mut self, string: &str) -> io::Result<()> {
-        self.empty = false;
-        writeln!(self.writer, "{}", string.red())
-    }
-}
diff --git a/pkgs/test/nixpkgs-check-by-name/src/validation.rs b/pkgs/test/nixpkgs-check-by-name/src/validation.rs
new file mode 100644
index 00000000000..e7279385152
--- /dev/null
+++ b/pkgs/test/nixpkgs-check-by-name/src/validation.rs
@@ -0,0 +1,102 @@
+use crate::nixpkgs_problem::NixpkgsProblem;
+use itertools::concat;
+use itertools::{
+    Either::{Left, Right},
+    Itertools,
+};
+use Validation::*;
+
+/// The validation result of a check.
+/// Instead of exiting at the first failure,
+/// this type can accumulate multiple failures.
+/// This can be achieved using the functions `and`, `sequence` and `sequence_`
+///
+/// This leans on https://hackage.haskell.org/package/validation
+pub enum Validation<A> {
+    Failure(Vec<NixpkgsProblem>),
+    Success(A),
+}
+
+impl<A> From<NixpkgsProblem> for Validation<A> {
+    /// Create a `Validation<A>` from a single check problem
+    fn from(value: NixpkgsProblem) -> Self {
+        Failure(vec![value])
+    }
+}
+
+/// A type alias representing the result of a check, either:
+/// - Err(anyhow::Error): A fatal failure, typically I/O errors.
+///   Such failures are not caused by the files in Nixpkgs.
+///   This hints at a bug in the code or a problem with the deployment.
+/// - Ok(Failure(Vec<NixpkgsProblem>)): A non-fatal validation problem with the Nixpkgs files.
+///   Further checks can be run even with this result type.
+///   Such problems can be fixed by changing the Nixpkgs files.
+/// - Ok(Success(A)): A successful (potentially intermediate) result with an arbitrary value.
+///   No fatal errors have occurred and no validation problems have been found with Nixpkgs.
+pub type Result<A> = anyhow::Result<Validation<A>>;
+
+pub trait ResultIteratorExt<A, E>: Sized + Iterator<Item = std::result::Result<A, E>> {
+    fn collect_vec(self) -> std::result::Result<Vec<A>, E>;
+}
+
+impl<I, A, E> ResultIteratorExt<A, E> for I
+where
+    I: Sized + Iterator<Item = std::result::Result<A, E>>,
+{
+    /// A convenience version of `collect` specialised to a vector
+    fn collect_vec(self) -> std::result::Result<Vec<A>, E> {
+        self.collect()
+    }
+}
+
+impl<A> Validation<A> {
+    /// Map a `Validation<A>` to a `Validation<B>` by applying a function to the
+    /// potentially contained value in case of success.
+    pub fn map<B>(self, f: impl FnOnce(A) -> B) -> Validation<B> {
+        match self {
+            Failure(err) => Failure(err),
+            Success(value) => Success(f(value)),
+        }
+    }
+}
+
+impl Validation<()> {
+    /// Combine two validations, both of which need to be successful for the return value to be successful.
+    /// The `NixpkgsProblem`s of both sides are returned concatenated.
+    pub fn and<A>(self, other: Validation<A>) -> Validation<A> {
+        match (self, other) {
+            (Success(_), Success(right_value)) => Success(right_value),
+            (Failure(errors), Success(_)) => Failure(errors),
+            (Success(_), Failure(errors)) => Failure(errors),
+            (Failure(errors_l), Failure(errors_r)) => Failure(concat([errors_l, errors_r])),
+        }
+    }
+}
+
+/// Combine many validations into a single one.
+/// All given validations need to be successful in order for the returned validation to be successful,
+/// in which case the returned validation value contains a `Vec` of each individual value.
+/// Otherwise the `NixpkgsProblem`s of all validations are returned concatenated.
+pub fn sequence<A>(check_results: impl IntoIterator<Item = Validation<A>>) -> Validation<Vec<A>> {
+    let (errors, values): (Vec<Vec<NixpkgsProblem>>, Vec<A>) = check_results
+        .into_iter()
+        .partition_map(|validation| match validation {
+            Failure(err) => Left(err),
+            Success(value) => Right(value),
+        });
+
+    // To combine the errors from the results we flatten all the error Vec's into a new Vec
+    // This is not very efficient, but doesn't matter because generally we should have no errors
+    let flattened_errors = errors.into_iter().concat();
+
+    if flattened_errors.is_empty() {
+        Success(values)
+    } else {
+        Failure(flattened_errors)
+    }
+}
+
+/// Like `sequence`, but without any containing value, for convenience
+pub fn sequence_(validations: impl IntoIterator<Item = Validation<()>>) -> Validation<()> {
+    sequence(validations).map(|_| ())
+}