Typeclass instance function type declarations (#5676) · Issues · Glasgow Haskell Compiler / GHC

Typeclass instance function type declarations

See discussion at http://stackoverflow.com/questions/8367426/why-cant-one-put-type-signatures-in-instance-declarations-in-haskell

Basically, it would be nice to be able to do something like this:

instance Functor Maybe where
  fmap :: (a -> b) -> Maybe a -> Maybe b
  fmap = undefined

This currently produces a "misplaced type signature" error.

It seems like a simple enough extension (we could call it SuperfluousTypeclassSignatures, because "Superfluous" is a cool word). It might be a good idea for Haskell Prime, but let's test it out as a Glasgow extension first.

We (a few of us on #haskell irc right now) think it best that only the precisely correct type signatures be accepted (reject signatures that are too lose or too strong), and it would be nice if, given an incorrect type signature, a custom error message were displayed stating the correct type signature.

Trac metadata

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

See discussion at http://stackoverflow.com/questions/8367426/why-cant-one-put-type-signatures-in-instance-declarations-in-haskell

Basically, it would be nice to be able to do something like this:

```
instance Functor Maybe where
  fmap :: (a -> b) -> Maybe a -> Maybe b
  fmap = undefined
```

This currently produces a "misplaced type signature" error.

We (a few of us on \#haskell irc right now) think it best that only the precisely correct type signatures be accepted (reject signatures that are too lose or too strong), and it would be nice if, given an incorrect type signature, a custom error message were displayed stating the correct type signature.

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

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

</details>