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Change "while evaluating <fun>" to "while *calling*" in trace

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The old way was not correct.

Here is an example:
```
 $ nix-instantiate --eval --expr 'let x = a: throw "asdf"; in x 1' --show-trace
error: asdf

       … while evaluating 'x'

       at «string»:1:9:

            1| let x = a: throw "asdf"; in x 1
             |         ^

       … from call site

       at «string»:1:29:

            1| let x = a: throw "asdf"; in x 1
             |                             ^
```
and yet also:
```
 $ nix-instantiate --eval --expr 'let x = a: throw "asdf"; in x' --show-trace
<LAMBDA>
```

Here is the thing: in both cases we are evaluating `x`!

Nix is a higher-order languages, and functions are a sort of value. When
we write `x = a: ...`, `a: ...` is the expression that `x` is being
defined to be, and that is already a value. Therefore, we should *never*
get an trace that says "while evaluating `x`", because evaluating `a:
...` is *trival* and nothing happens during it!

What is actually happening here is we are applying `x` and evaluating
its *body* with arguments substituted for parameters. I think the
simplest way to say is just "while *calling* `x`", and so that is what I
changed it to.
This commit is contained in:
John Ericson 2022-12-02 09:00:03 -05:00
parent ca42068bdc
commit 19c5394971
1 changed files with 2 additions and 2 deletions
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4
src/libexpr/eval.cc
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@ -1646,7 +1646,7 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
auto dts = debugRepl
? makeDebugTraceStacker(
*this, *lambda.body, env2, positions[lambda.pos],
"while evaluating %s",
"while calling %s",
lambda.name
? concatStrings("'", symbols[lambda.name], "'")
: "anonymous lambda")
@ -1655,7 +1655,7 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
lambda.body->eval(*this, env2, vCur);
} catch (Error & e) {
if (loggerSettings.showTrace.get()) {
addErrorTrace(e, lambda.pos, "while evaluating %s",
addErrorTrace(e, lambda.pos, "while calling %s",
(lambda.name
? concatStrings("'", symbols[lambda.name], "'")
: "anonymous lambda"));