Provide way to show the origin of a constraint

For a complex type (A b, C d, E e) => Something a b e -> Int, provide a way to given the query: where does A b come from? Respond with the line number of a function that causes that constraint. This should of course also work for non-Haskell 98 constraints.

This issue comes up when one by accident calls a function in the wrong layer of a monadic transformer stack.

Trac metadata

Trac field	Value
Version	6.6.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Unknown
Architecture	Unknown

changed weight to 5

added Tfeature request Trac import labels

If you could give a concrete example of what you have in mind, it'd help. That is, exactly what do you want the programmer to be able to do or say? GHC does indeed report the origins for constraints in error messages, for example.

S

It sounds hard have an interface for asking the question (perhaps it's not too bad inside ghci), hard to give the answer that the user is after, and would probably require significant changes to the typechecker, so I'm afraid I don't think we'll get to this any time soon.

Trac metadata

Trac field	Value
Component	Compiler → Compiler (Type checker)

added typechecker label

Trac metadata

Trac field	Value
Architecture	Unknown → Unknown/Multiple

Trac metadata

Trac field	Value
Operating system	Unknown → Unknown/Multiple

Bump; still an interesting feature request?

Trac metadata

Trac field	Value
TypeOfFailure	- → OtherFailure

added Pnormal label

mentioned in merge request !11058 (merged)