Error message prints invisible kind arguments in a visible matter
Consider the following program:
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TypeInType #-}
module Bug where
import Data.Kind
import Data.Proxy
data T :: forall a. a -> Type
f1 :: Proxy (T True)
f1 = "foo"
f2 :: forall (t :: forall a. a -> Type).
Proxy (t True)
f2 = "foo"This program doesn't typecheck (for good reason). The error messages, however, are a bit iffy:
$ /opt/ghc/8.4.3/bin/ghc Bug.hs
[1 of 1] Compiling Bug ( Bug.hs, Bug.o )
Bug.hs:11:6: error:
• Couldn't match expected type ‘Proxy (T 'True)’
with actual type ‘[Char]’
• In the expression: "foo"
In an equation for ‘f1’: f1 = "foo"
|
11 | f1 = "foo"
| ^^^^^
Bug.hs:15:6: error:
• Couldn't match expected type ‘Proxy (t Bool 'True)’
with actual type ‘[Char]’
• In the expression: "foo"
In an equation for ‘f2’: f2 = "foo"
• Relevant bindings include
f2 :: Proxy (t Bool 'True) (bound at Bug.hs:15:1)
|
15 | f2 = "foo"
| ^^^^^Note that in the error message for f1, the type T 'True is printed correctly—the invisible Bool argument is omitted. However, in the error message for f2, this is not the case, as the type t Bool 'True is printed. That Bool is an invisible argument as well, and should not be printed without the use of, say, -fprint-explicit-kinds.
Trac metadata
| Trac field | Value |
|---|---|
| Version | 8.4.3 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler (Type checker) |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture |