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Improve error message for fields called `this`, `self` or `arg` in templates or template choices
Currently, declaring fields with these names results in poor error messages, for example,
template Foo
with
p : Party
where
signatory p
choice FooC : ()
with
arg : Decimal
controller p
do
return ()
fails with
Foo.daml:41:18: error:
• Conflicting definitions for ‘arg’
Bound at: Foo.daml:41:18
<no location info>
• In a lambda abstraction
Foo.daml:42:7: error:
• Conflicting definitions for ‘arg’
Bound at: Foo.daml:(42,7)-(43,17)
<no location info>
• In a lambda abstraction
Ideally, this would be caught by the daml-preprocessor
, giving users an actionable error message.
Background
This was originally reported as https://github.com/digital-asset/daml/issues/3017, but the idea there was to allow such fields by tweaking the desugaring. That approach introduces a lot of complexity for little gain, so instead we just want to improve the error messages.
Might be easier to forbid arg etc as a variable name in this context and propagate this as an error message.
We'd need to specifically check choice/argument fields, and any body that can take this, self or arg as an argument. This sounds like it could get messy, I think a daml suggested warning would be better
Alternatively, we can simply search template/choice arguments to look for these, and not generate the argument label in desugaring if it exists. Maybe alongside a warning for using this name