Opened 3 months ago

Last modified 4 weeks ago

#14180 new bug

Strange/bad error message binding unboxed type variable

Reported by: dfeuer Owned by:
Priority: normal Milestone: 8.2.3
Component: Compiler (Type checker) Version: 8.3
Keywords: TypeInType Cc: goldfire
Operating System: Unknown/Multiple Architecture: Unknown/Multiple
Type of failure: None/Unknown Test Case:
Blocked By: Blocking:
Related Tickets: Differential Rev(s):
Wiki Page:

Description

{-# language TypeInType, TypeFamilies, MagicHash #-}

import GHC.Exts

type family MatchInt (f :: Int) :: () where
  MatchInt ('I# x) = '()

produces

<interactive>:2:59: error:
    • Couldn't match a lifted type with an unlifted type
      When matching kinds
        k0 :: *
        Int# :: TYPE 'IntRep
      Expected kind ‘Int#’, but ‘x’ has kind ‘k0’
    • In the first argument of ‘ 'I#’, namely ‘x’
      In the first argument of ‘MatchInt’, namely ‘( 'I# x)’
      In the type family declaration for ‘MatchInt’

If, however, I replace x in the pattern with _, the type checker is satisfied. If I give it a pattern signature,

MatchInt ('I# (x :: Int#)) = '()

I get a different (and equally unhelpful) error message,

    • Expecting a lifted type, but ‘Int#’ is unlifted
    • In the kind ‘Int#’
      In the first argument of ‘ 'I#’, namely ‘(x :: Int#)’
      In the first argument of ‘MatchInt’, namely ‘( 'I# (x :: Int#))’

Change History (4)

comment:1 Changed 3 months ago by dfeuer

Cc: goldfire added

comment:2 Changed 3 months ago by simonpj

What do you expect to happen here? We just don't have unboxed ints at the type level. Doubtless the error message could be improved.

comment:3 in reply to:  2 Changed 3 months ago by dfeuer

Replying to simonpj:

What do you expect to happen here? We just don't have unboxed ints at the type level. Doubtless the error message could be improved.

Well, a message to that effect would be much better than what we get now! But I don't have a sufficiently clear sense of exactly what is and is not allowed to write it myself. Something about variables bound in type families not being able to have kinds of kind TYPE r unless r ~ PtrRepLifted, perhaps?

comment:4 Changed 4 weeks ago by bgamari

Milestone: 8.2.28.2.3

It looks like this won't be fixed for 8.2.2.

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