TH can sneak in TypeApplications without enabling the extension (#16133) · Issues · Glasgow Haskell Compiler / GHC

TH can sneak in TypeApplications without enabling the extension

This program compiles:

{-# LANGUAGE TemplateHaskell #-}
module Bug1 where

import Language.Haskell.TH

$([d| f :: Int
      f = $(varE 'id `appTypeE` conT ''Int `appE` litE (integerL 42)) |])

But it shouldn't: this splices in code that uses TypeApplications, but the language extension isn't enabled! What's worse, this is inconsistent with visible kind applications, since the following code //is// rejected:

{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TemplateHaskell #-}
module Bug2 where

import Data.Kind
import Language.Haskell.TH hiding (Type)

data P (a :: k) = MkP

$([d| type P' = $(conT ''P `appKindT` conT ''Type) |])

$ ~/Software/ghc4/inplace/bin/ghc-stage2 Bug2.hs
[1 of 1] Compiling Bug2             ( Bug2.hs, Bug2.o )

Bug2.hs:10:3: error:
    Illegal visible kind application ‘Type’
      Perhaps you intended to use TypeApplications
   |
10 | $([d| type P' = $(conT ''P `appKindT` conT ''Type) |])
   |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Let's bring the two into harmony. Patch incoming.

Trac metadata

Trac field	Value
Version	8.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Template Haskell
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

This program compiles:

```hs
{-# LANGUAGE TemplateHaskell #-}
module Bug1 where

import Language.Haskell.TH

$([d| f :: Int
      f = $(varE 'id `appTypeE` conT ''Int `appE` litE (integerL 42)) |])
```

But it shouldn't: this splices in code that uses `TypeApplications`, but the language extension isn't enabled! What's worse, this is inconsistent with visible kind applications, since the following code //is// rejected:

```hs
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TemplateHaskell #-}
module Bug2 where

import Data.Kind
import Language.Haskell.TH hiding (Type)

data P (a :: k) = MkP

$([d| type P' = $(conT ''P `appKindT` conT ''Type) |])
```

```
$ ~/Software/ghc4/inplace/bin/ghc-stage2 Bug2.hs
[1 of 1] Compiling Bug2             ( Bug2.hs, Bug2.o )

Bug2.hs:10:3: error:
    Illegal visible kind application ‘Type’
      Perhaps you intended to use TypeApplications
   |
10 | $([d| type P' = $(conT ''P `appKindT` conT ''Type) |])
   |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```

Let's bring the two into harmony. Patch incoming.

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

| Trac field             | Value            |
| ---------------------- | ---------------- |
| Version                | 8.6.3            |
| Type                   | Bug              |
| TypeOfFailure          | OtherFailure     |
| Priority               | normal           |
| Resolution             | Unresolved       |
| Component              | Template Haskell |
| Test case              |                  |
| Differential revisions |                  |
| BlockedBy              |                  |
| Related                |                  |
| Blocking               |                  |
| CC                     |                  |
| Operating system       |                  |
| Architecture           |                  |

</details>