Kind variable scoping problem in Iface file when using Template Haskell (#7022) · Issues · Glasgow Haskell Compiler / GHC

Kind variable scoping problem in Iface file when using Template Haskell

Consider the following modules:

module PolykindTH where

import Language.Haskell.TH

makeSList :: Q [Dec]
makeSList = do
  a <- newName "a"
  k <- newName "k"
  return [TySynD (mkName "SList") [KindedTV a (AppT ListT (VarT k))]
                 (AppT (ConT (mkName "Sing")) (VarT a))]
-- makes "type SList (a :: [k]) = Sing a"

{-# LANGUAGE PolyKinds, TypeFamilies, DataKinds, TemplateHaskell #-}

module Polykind where

import PolykindTH

data family Sing (a :: k)

$( makeSList )

When these modules are compiled, the resultant Iface file fails to load when code attempts to use the modules. To check this, I packaged this up with a cabal file, registered the package with my GHC (7.5.20120620), and then attempted to compile the following:

import Polykind

foo :: SList a -> Bool
foo = undefined

Here is the error:

Declaration for SList:
  Iface type variable out of scope:  k
Cannot continue after interface file error

I have attached a tarball of my cabal package for ease of testing.

Trac metadata

Trac field	Value
Version	7.5
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

Consider the following modules:

```
module PolykindTH where

import Language.Haskell.TH

makeSList :: Q [Dec]
makeSList = do
  a <- newName "a"
  k <- newName "k"
  return [TySynD (mkName "SList") [KindedTV a (AppT ListT (VarT k))]
                 (AppT (ConT (mkName "Sing")) (VarT a))]
-- makes "type SList (a :: [k]) = Sing a"
```

```
{-# LANGUAGE PolyKinds, TypeFamilies, DataKinds, TemplateHaskell #-}

module Polykind where

import PolykindTH

data family Sing (a :: k)

$( makeSList )
```

```
import Polykind

foo :: SList a -> Bool
foo = undefined
```

Here is the error:

```
Declaration for SList:
  Iface type variable out of scope:  k
Cannot continue after interface file error
```

I have attached a tarball of my cabal package for ease of testing.

<details><summary>Trac metadata</summary>

| Trac field             | Value        |
| ---------------------- | ------------ |
| Version                | 7.5          |
| Type                   | Bug          |
| TypeOfFailure          | OtherFailure |
| Priority               | normal       |
| Resolution             | Unresolved   |
| Component              | Compiler     |
| Test case              |              |
| Differential revisions |              |
| BlockedBy              |              |
| Related                |              |
| Blocking               |              |
| CC                     |              |
| Operating system       |              |
| Architecture           |              |

</details>