Opened 19 months ago

Closed 7 months ago

## #11966 closed bug (fixed)

# Surprising behavior with higher-rank quantification of kind variables

Reported by: | ocharles | Owned by: | |
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Priority: | normal | Milestone: | 8.2.1 |

Component: | Compiler (Type checker) | Version: | 8.0.1-rc3 |

Keywords: | TypeInType | Cc: | |

Operating System: | Unknown/Multiple | Architecture: | Unknown/Multiple |

Type of failure: | GHC rejects valid program | Test Case: | dependent/should_compile/T11966 |

Blocked By: | Blocking: | ||

Related Tickets: | Differential Rev(s): | ||

Wiki Page: |

### Description

Sorry about the rubbish title. I wrote the following code, which type checks:

{-# LANGUAGE TypeFamilies #-} {-# LANGUAGE DataKinds #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE PolyKinds #-} {-# LANGUAGE RankNTypes #-} {-# LANGUAGE TypeInType #-} {-# LANGUAGE TypeOperators #-} module Test where import Data.Kind (Type) -- Simplification type family Col (f :: k -> j) (x :: k) :: Type -- Base types data PGBaseType = PGInteger | PGText -- Transformations data Column t = Column Symbol t newtype Nullable t = Nullable t newtype HasDefault t = HasDefault t -- Interpretations data Expr k data Record (f :: forall k. k -> Type) = Record {rX :: Col f ('Column "x" 'PGInteger) ,rY :: Col f ('Column "y" ('Nullable 'PGInteger)) ,rZ :: Col f ('HasDefault 'PGText)}

However, if I play with this in GHCI, I can get the following error:

λ> let x = undefined :: Record Expr <interactive>:136:29-32: error: • Expected kind ‘forall k. k -> Type’, but ‘Expr’ has kind ‘forall k. k -> *’ • In the first argument of ‘Record’, namely ‘Expr’ In an expression type signature: Record Expr In the expression: undefined :: Record Expr

It seems that if I write

data Expr :: forall k. k -> Type

things work, but I'm unclear if/why that is necessary, and the error message certainly needs to be fixed.

### Change History (3)

### comment:1 Changed 7 months ago by

### comment:3 Changed 7 months ago by

Milestone: | → 8.2.1 |
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Resolution: | → fixed |

Status: | new → closed |

Test Case: | → dependent/should_compile/T11966 |

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Luckily, commit a7ee2d4c4229b27af324ebac93081f692835365d (Improve TcFlatten.flattenTyVar) fixed this! I'll add a regression test.