Opened 6 months ago

Last modified 6 months ago

#14238 new bug

`:kind` suppresses visible dependent quantifiers by default in GHCi 8.2.1

Reported by: RyanGlScott Owned by:
Priority: normal Milestone:
Component: Compiler (Type checker) Version: 8.2.1
Keywords: TypeInType Cc:
Operating System: Unknown/Multiple Architecture: Unknown/Multiple
Type of failure: Incorrect result at runtime Test Case:
Blocked By: Blocking:
Related Tickets: Differential Rev(s):
Wiki Page:


Load this program into GHCi on 8.2.1 or later:

{-# LANGUAGE TypeInType #-}

import Data.Kind

data Foo (k :: Type) :: k -> Type where
  MkFoo :: Foo (k1 -> k2) f -> Foo k1 a -> Foo k2 (f a)

And ask it what the kind of Foo is:

GHCi, version 8.2.1:  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Main             ( Bug.hs, interpreted )
Ok, 1 module loaded.
λ> :k Foo
Foo :: k -> *

This is just plain wrong: the actual kind of Foo should be forall k -> k -> *. Normally, one can omit foralls from a kind signature, but this is a special case where we're using a visible forall. In other words, forall k -> k -> * is not the same as k -> *, as the former takes two arguments, whereas the latter takes one.

A workaround is to force GHCi to come to its senses by explicitly enabling -fprint-explicit-foralls:

λ> :set -fprint-explicit-foralls 
λ> :k Foo
Foo :: forall k -> k -> *

This is actually a regression since GHC 8.0.2, since GHCi did the right thing by default then:

GHCi, version 8.0.2:  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Main             ( Bug.hs, interpreted )
Ok, modules loaded: Main.
λ> :k Foo
Foo :: forall k -> k -> Type

Change History (2)

comment:1 Changed 6 months ago by simonpj

I agree!

For terms we have a difference between :info (which takes a name) and :type (which takes an expression, and may involve instantiating and re-generalising).

comment:2 Changed 6 months ago by goldfire

Agreed as well. The ticket has TypeInType on it already, and so I'll get to it in due course, unless someone grabs it sooner.

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