Improve an error message when an ability variable can't be solved. #5825
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In definitions like:
foo : '{g} a -> ()
foo th =
!th
foo th
we end up in a situation where we can't actually solve to the type
`'{g} a ->{g} ()`. The reason is that we make up the existential
variable immediately so that we can type check the recursive function
with a specified type of `'{g} a ->{e} ()`. The `g` that `e` needs to
be solved to ends up later in the context, so the solution is not
allowed.
This change introduces a new error message that detects this situation
and suggests an ability set for the signature. There was a previous
'pretty' error message that had been disabled for this situation, but
it was fairly uninformative, and might have been just as useless as
the ugly error message we were currently displaying.
|
Nice! Might be helpful to capture it in a failing block in a transcript to avoid accidental regressions to the error message. |
Yes please — for me it's mainly to have an example of each error message in the transcript output somewhere. |
|
Added. |
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In definitions like:
we end up in a situation where we can't actually solve to the type
'{g} a ->{g} (). The reason is that we make up the existential variable immediately so that we can type check the recursive function with a specified type of'{g} a ->{e} (). Thegthateneeds to be solved to ends up later in the context, so the solution is not allowed.This change introduces a new error message that detects this situation and suggests an ability set for the signature. There was a previous 'pretty' error message that had been disabled for this situation, but it was fairly uninformative, and might have been just as useless as the ugly error message we were currently displaying.
Fixes #5655 and #5025
Note: this problem only occurs with recursive definitions. We actually take steps to avoid this problem in other situations (do
existentializeArrowsafter unbinding the foralls). But this isn't an option for recursive let, because the control flow adds the existentials and type signatures to the context before checking any definitions in the binding group.