Poor error message source location reporting with unsaturated type family
If you compile this program:
{-# LANGUAGE TypeFamilies #-}
module Bug where
newtype WrapChar f = MkWrapChar (f Char)
type family F a
type family T a
type instance T Int = WrapChar FThen you'll get this error message:
$ /opt/ghc/8.6.2/bin/ghc Bug.hs
[1 of 1] Compiling Bug ( Bug.hs, Bug.o )
Bug.hs:7:15: error:
• The type family ‘F’ should have 1 argument, but has been given none
• In the type instance declaration for ‘T’
|
7 | type instance T Int = WrapChar F
| ^The thing is, the location of that caret is rather unfocused. It's pointing at T, but the real culprit is F!
I know that GHC can do better here because other error messages actually get this right. For instance, if you change the T Int instance to this:
type instance T Int = FThen GHC will actually point to F in the resulting error message:
$ /opt/ghc/8.6.2/bin/ghc Bug.hs
[1 of 1] Compiling Bug ( Bug.hs, Bug.o )
Bug.hs:7:23: error:
• Expecting one more argument to ‘F’
Expected a type, but ‘F’ has kind ‘* -> *’
• In the type ‘F’
In the type instance declaration for ‘T’
|
7 | type instance T Int = F
| ^If GHC can point to the right place in the source code in this situation, then it ought to get the earlier situation right as well.
Trac metadata
| Trac field | Value |
|---|---|
| Version | 8.6.2 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture |