Merge pull request #7158 from sternenseemann/foldl-strict-accumulation-value

tests/lang/eval-fail-foldlStrict-strict-op-application.nix

@@ -0,0 +1,5 @@
+# Tests that the result of applying op is forced even if the value is never used
+builtins.foldl'
+  (_: f: f null)
+  null
+  [ (_: throw "Not the final value, but is still forced!") (_: 23) ]

tests/lang/eval-okay-foldlStrict-lazy-elements.exp

@@ -0,0 +1 @@
+42

tests/lang/eval-okay-foldlStrict-lazy-elements.nix

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+# Tests that the rhs argument of op is not forced unconditionally
+let
+  lst = builtins.foldl'
+    (acc: x: acc ++ [ x ])
+    [ ]
+    [ 42 (throw "this shouldn't be evaluated") ];
+in
+
+builtins.head lst

tests/lang/eval-okay-foldlStrict-lazy-initial-accumulator.exp

@@ -0,0 +1 @@
+42

tests/lang/eval-okay-foldlStrict-lazy-initial-accumulator.nix

@@ -0,0 +1,6 @@
+# Checks that the nul value for the accumulator is not forced unconditionally.
+# Some languages provide a foldl' that is strict in this argument, but Nix does not.
+builtins.foldl'
+  (_: x: x)
+  (throw "This is never forced")
+  [ "but the results of applying op are" 42 ]