1 files changed, 46 insertions, 8 deletions

diff --git a/lib/debug.nix b/lib/debug.nix
index 2879f72ed2b..e3ca3352397 100644
--- a/lib/debug.nix
+++ b/lib/debug.nix
@@ -14,9 +14,25 @@
 */
 { lib }:
 let
-  inherit (builtins) trace isAttrs isList isInt
-          head substring attrNames;
-  inherit (lib) id elem isFunction;
+  inherit (lib)
+    isInt
+    attrNames
+    isList
+    isAttrs
+    substring
+    addErrorContext
+    attrValues
+    concatLists
+    concatStringsSep
+    const
+    elem
+    generators
+    head
+    id
+    isDerivation
+    isFunction
+    mapAttrs
+    trace;
 in
 
 rec {
@@ -94,7 +110,7 @@ rec {
        trace: { a = { b = {…}; }; }
        => null
    */
-  traceSeqN = depth: x: y: with lib;
+  traceSeqN = depth: x: y:
     let snip = v: if      isList  v then noQuotes "[…]" v
                   else if isAttrs v then noQuotes "{…}" v
                   else v;
@@ -132,6 +148,28 @@ rec {
   /* A combination of `traceVal` and `traceSeqN`. */
   traceValSeqN = traceValSeqNFn id;
 
+  /* Trace the input and output of a function `f` named `name`,
+  both down to `depth`.
+
+  This is useful for adding around a function call,
+  to see the before/after of values as they are transformed.
+
+     Example:
+       traceFnSeqN 2 "id" (x: x) { a.b.c = 3; }
+       trace: { fn = "id"; from = { a.b = {…}; }; to = { a.b = {…}; }; }
+       => { a.b.c = 3; }
+  */
+  traceFnSeqN = depth: name: f: v:
+    let res = f v;
+    in lib.traceSeqN
+        (depth + 1)
+        {
+          fn = name;
+          from = v;
+          to = res;
+        }
+        res;
+
 
   # -- TESTING --
 
@@ -149,7 +187,7 @@ rec {
   */
   runTests =
     # Tests to run
-    tests: lib.concatLists (lib.attrValues (lib.mapAttrs (name: test:
+    tests: concatLists (attrValues (mapAttrs (name: test:
     let testsToRun = if tests ? tests then tests.tests else [];
     in if (substring 0 4 name == "test" ||  elem name testsToRun)
        && ((testsToRun == []) || elem name tests.tests)
@@ -176,9 +214,9 @@ rec {
           + "and will be removed in the next release. "
           + "Please use more specific concatenation "
           + "for your uses (`lib.concat(Map)StringsSep`)." )
-    (lib.concatStringsSep "; " (map (x: "${x}=") (attrNames a)));
+    (concatStringsSep "; " (map (x: "${x}=") (attrNames a)));
 
-  showVal = with lib;
+  showVal =
     trace ( "Warning: `showVal` is deprecated "
           + "and will be removed in the next release, "
           + "please use `traceSeqN`" )
@@ -226,7 +264,7 @@ rec {
     trace ( "Warning: `addErrorContextToAttrs` is deprecated "
           + "and will be removed in the next release. "
           + "Please use `builtins.addErrorContext` directly." )
-    (lib.mapAttrs (a: v: lib.addErrorContext "while evaluating ${a}" v) attrs);
+    (mapAttrs (a: v: addErrorContext "while evaluating ${a}" v) attrs);
 
   # example: (traceCallXml "myfun" id 3) will output something like
   # calling myfun arg 1: 3 result: 3