Type signatures in patterns not (yet) handled by Template Haskell (#12164) · Issues · Glasgow Haskell Compiler / GHC

Type signatures in patterns not (yet) handled by Template Haskell

This bit me just now, so I figured I should report a feature request bug.

ezyang@sabre:~/Dev/whisk$ ghc-8.0 --interactive -XTemplateHaskell
GHCi, version 8.0.0.20160204: http://www.haskell.org/ghc/  :? for help
Prelude> Language.Haskell.TH.runQ [| \(x :: ()) -> x |]

<interactive>:1:29: error:
    Type signatures in patterns not (yet) handled by Template Haskell
      x :: ()

In the code, there is a bit of mumbling about scoped type variables. I believe it is referring to code like this:

Prelude> :t (\(x :: a) -> (x :: a))
(\(x :: a) -> (x :: a)) :: a -> a

Described in the manual here: https://downloads.haskell.org/\~ghc/latest/docs/html/users_guide/glasgow_exts.html\#pattern-type-signatures This means that TH has to somehow bring the type into scope. I don't actually know how to do it.

It seems a bit awful for this to be the reason why these quotes are not accepted; maybe without scoped type variables we should be able to do something cheap and cheerful.

Trac metadata

Trac field	Value
Version	8.0.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Template Haskell
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

This bit me just now, so I figured I should report a feature request bug.

```
ezyang@sabre:~/Dev/whisk$ ghc-8.0 --interactive -XTemplateHaskell
GHCi, version 8.0.0.20160204: http://www.haskell.org/ghc/  :? for help
Prelude> Language.Haskell.TH.runQ [| \(x :: ()) -> x |]

<interactive>:1:29: error:
    Type signatures in patterns not (yet) handled by Template Haskell
      x :: ()
```

In the code, there is a bit of mumbling about scoped type variables. I believe it is referring to code like this:

```
Prelude> :t (\(x :: a) -> (x :: a))
(\(x :: a) -> (x :: a)) :: a -> a
```

Described in the manual here: https://downloads.haskell.org/\~ghc/latest/docs/html/users_guide/glasgow_exts.html\#pattern-type-signatures
This means that TH has to somehow bring the type into scope. I don't actually know how to do it.

It seems a bit awful for this to be the reason why these quotes are not accepted; maybe without scoped type variables we should be able to do something cheap and cheerful.

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

| Trac field             | Value            |
| ---------------------- | ---------------- |
| Version                | 8.0.1            |
| Type                   | FeatureRequest   |
| TypeOfFailure          | OtherFailure     |
| Priority               | normal           |
| Resolution             | Unresolved       |
| Component              | Template Haskell |
| Test case              |                  |
| Differential revisions |                  |
| BlockedBy              |                  |
| Related                |                  |
| Blocking               |                  |
| CC                     |                  |
| Operating system       |                  |
| Architecture           |                  |

</details>