From 81499a0b93a136f889f3799d7110dcc479a4cbe1 Mon Sep 17 00:00:00 2001
From: Maximilian Bosch <maximilian@mbosch.me>
Date: Sat, 20 Jan 2024 16:05:30 +0100
Subject: [PATCH] libexpr: print value of what is attempted to be called as
 function
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Low-hanging fruit in the spirit of #9753 and #9754 (means 9999years did
all the hard work already).

This basically prints out what was attempted to be called as function,
i.e.

  map (import <nixpkgs> {}) [ 1 2 3 ]

now gives the following error message:

    error:
           … while calling the 'map' builtin
             at «string»:1:1:
                1| map (import <nixpkgs> {}) [ 1 2 3 ]
                 | ^

           … while evaluating the first argument passed to builtins.map

           error: expected a function but found a set: { _type = "pkgs"; AAAAAASomeThingsFailToEvaluate = «thunk»; AMB-plugins = «thunk»; ArchiSteamFarm = «thunk»; BeatSaberModManager = «thunk»; CHOWTapeModel = «thunk»; ChowCentaur = «thunk»; ChowKick = «thunk»; ChowPhaser = «thunk»; CoinMP = «thunk»;  «18783 attributes elided»}
---
 src/libexpr/eval.cc                | 6 +++++-
 tests/unit/libexpr/error_traces.cc | 4 ++--
 2 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 71e956e10..ce410162e 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -1692,7 +1692,11 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
         }
 
         else
-            error("attempt to call something which is not a function but %1%", showType(vCur)).atPos(pos).debugThrow<TypeError>();
+            error("attempt to call something which is not a function but %1%: %2%",
+                    showType(vCur),
+                    ValuePrinter(*this, vCur, errorPrintOptions))
+                .atPos(pos)
+                .debugThrow<TypeError>();
     }
 
     vRes = vCur;
diff --git a/tests/unit/libexpr/error_traces.cc b/tests/unit/libexpr/error_traces.cc
index f0cad58bb..f99aafd74 100644
--- a/tests/unit/libexpr/error_traces.cc
+++ b/tests/unit/libexpr/error_traces.cc
@@ -750,7 +750,7 @@ namespace nix {
 
         ASSERT_TRACE1("foldl' (_: 1) \"foo\" [ true ]",
                       TypeError,
-                      hintfmt("attempt to call something which is not a function but %s", "an integer"));
+                      hintfmt("attempt to call something which is not a function but %s: %s", "an integer", ANSI_CYAN "1" ANSI_NORMAL));
 
         ASSERT_TRACE2("foldl' (a: b: a && b) \"foo\" [ true ]",
                       TypeError,
@@ -835,7 +835,7 @@ namespace nix {
 
         ASSERT_TRACE1("sort (_: 1) [ \"foo\" \"bar\" ]",
                       TypeError,
-                      hintfmt("attempt to call something which is not a function but %s", "an integer"));
+                      hintfmt("attempt to call something which is not a function but %s: %s", "an integer", ANSI_CYAN "1" ANSI_NORMAL));
 
         ASSERT_TRACE2("sort (_: _: 1) [ \"foo\" \"bar\" ]",
                       TypeError,