Unhelpful names for wildcard type variables (#16203) · Issues · Glasgow Haskell Compiler / GHC

Unhelpful names for wildcard type variables

Consider

{-# LANGUAGE PartialTypeSignatures #-}

f :: _ -> _
f x = x

With GHC 8.6 we got

T16152.hs:10:6: warning: [-Wpartial-type-signatures]
    • Found type wildcard ‘_’ standing for ‘w’
      Where: ‘w’ is a rigid type variable bound by
               the inferred type of f :: w -> w
               at T16152.hs:11:1-7
    • In the type signature: f :: _ -> _
   |
10 | f :: _ -> _
   |      ^

T16152.hs:10:11: warning: [-Wpartial-type-signatures]
    • Found type wildcard ‘_’ standing for ‘w’
      Where: ‘w’ is a rigid type variable bound by
               the inferred type of f :: w -> w
               at T16152.hs:11:1-7
    • In the type signature: f :: _ -> _
   |
10 | f :: _ -> _
   |           ^

But with HEAD we get

T16152.hs:10:6: warning: [-Wpartial-type-signatures]
    • Found type wildcard ‘_’ standing for ‘_’
      Where: ‘_’ is a rigid type variable bound by
               the inferred type of f :: _ -> _
               at T16152.hs:11:1-7
    • In the type ‘_ -> _’
      In the type signature: f :: _ -> _
   |
10 | f :: _ -> _
   |      ^

T16152.hs:10:11: warning: [-Wpartial-type-signatures]
    • Found type wildcard ‘_’ standing for ‘_’
      Where: ‘_’ is a rigid type variable bound by
               the inferred type of f :: _ -> _
               at T16152.hs:11:1-7
    • In the type ‘_ -> _’
      In the type signature: f :: _ -> _
   |
10 | f :: _ -> _
   |           ^

Saying "Found type wildcard ‘’ standing for ‘’" is unhelpful. The "w" form is much better.

The change is caused by

commit 17bd163566153babbf51adaff8397f948ae363ca
Author: mynguyen <mnguyen1@brynmawr.edu>
Date:   Tue Dec 18 11:52:26 2018 -0500

    Visible kind application

which has this patch

-newWildTyVar _name
+newWildTyVar
   = do { kind <- newMetaKindVar
        ; uniq <- newUnique
        ; details <- newMetaDetails TauTv
-       ; let name = mkSysTvName uniq (fsLit "w")
+       ; let name = mkSysTvName uniq (fsLit "_")
              tyvar = (mkTcTyVar name kind details)
        ; traceTc "newWildTyVar" (ppr tyvar)
        ; return tyvar }

What was the reason for this change? Can we change "_" back to "w" please?

Trac metadata

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

Consider

```
{-# LANGUAGE PartialTypeSignatures #-}

f :: _ -> _
f x = x
```

With GHC 8.6 we got

```
T16152.hs:10:6: warning: [-Wpartial-type-signatures]
    • Found type wildcard ‘_’ standing for ‘w’
      Where: ‘w’ is a rigid type variable bound by
               the inferred type of f :: w -> w
               at T16152.hs:11:1-7
    • In the type signature: f :: _ -> _
   |
10 | f :: _ -> _
   |      ^

T16152.hs:10:11: warning: [-Wpartial-type-signatures]
    • Found type wildcard ‘_’ standing for ‘w’
      Where: ‘w’ is a rigid type variable bound by
               the inferred type of f :: w -> w
               at T16152.hs:11:1-7
    • In the type signature: f :: _ -> _
   |
10 | f :: _ -> _
   |           ^
```

But with HEAD we get

```
T16152.hs:10:6: warning: [-Wpartial-type-signatures]
    • Found type wildcard ‘_’ standing for ‘_’
      Where: ‘_’ is a rigid type variable bound by
               the inferred type of f :: _ -> _
               at T16152.hs:11:1-7
    • In the type ‘_ -> _’
      In the type signature: f :: _ -> _
   |
10 | f :: _ -> _
   |      ^

T16152.hs:10:11: warning: [-Wpartial-type-signatures]
    • Found type wildcard ‘_’ standing for ‘_’
      Where: ‘_’ is a rigid type variable bound by
               the inferred type of f :: _ -> _
               at T16152.hs:11:1-7
    • In the type ‘_ -> _’
      In the type signature: f :: _ -> _
   |
10 | f :: _ -> _
   |           ^
```

Saying "Found type wildcard ‘_’ standing for ‘_’" is unhelpful. The "w" form is much better.

The change is caused by

```
commit 17bd163566153babbf51adaff8397f948ae363ca
Author: mynguyen <mnguyen1@brynmawr.edu>
Date:   Tue Dec 18 11:52:26 2018 -0500

Visible kind application
```

which has this patch

```
-newWildTyVar _name
+newWildTyVar
   = do { kind <- newMetaKindVar
        ; uniq <- newUnique
        ; details <- newMetaDetails TauTv
-       ; let name = mkSysTvName uniq (fsLit "w")
+       ; let name = mkSysTvName uniq (fsLit "_")
              tyvar = (mkTcTyVar name kind details)
        ; traceTc "newWildTyVar" (ppr tyvar)
        ; return tyvar }
```

What was the reason for this change? Can we change "_" back to "w" please?

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

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

</details>