Allow unsaturated uses of unlifted types in Core (#11736) · Issues · Glasgow Haskell Compiler / GHC

Allow unsaturated uses of unlifted types in Core

Currently Core Lint checks for unsaturated uses of unlifted types in . These are easily produced in the new type-indexed Typeable scheme. For instance, consider solving for Typeable # (Array# Int):

We decompose the application into wanteds Typeable (* -> #) Array# and Typeable * Int
We construct dictionaries for both, giving us a term typeRepArray# :: TypeRep (* -> #) Array#

While nothing seems to blow up with this patch,

diff --git a/compiler/coreSyn/CoreLint.hs b/compiler/coreSyn/CoreLint.hs
index 99625c9..2c401de 100644
--- a/compiler/coreSyn/CoreLint.hs
+++ b/compiler/coreSyn/CoreLint.hs
@@ -1040,7 +1040,7 @@ lintType ty@(TyConApp tc tys)
   = lintType ty'   -- Expand type synonyms, so that we do not bogusly complain
                    --  about un-saturated type synonyms
 
-  | isUnliftedTyCon tc || isTypeSynonymTyCon tc || isTypeFamilyTyCon tc
+  | isTypeSynonymTyCon tc || isTypeFamilyTyCon tc
        -- Also type synonyms and type families
   , length tys < tyConArity tc
   = failWithL (hang (text "Un-saturated type application") 2 (ppr ty))

I otherwise have no reason to believe that this is safe.

Trac metadata

Trac field	Value
Version	8.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	goldfire, simonpj
Operating system
Architecture

Currently Core Lint checks for unsaturated uses of unlifted types in . These are easily produced in the new type-indexed `Typeable` scheme. For instance, consider solving for `Typeable # (Array# Int)`:

- We decompose the application into wanteds `Typeable (* -> #) Array#` and `Typeable * Int`
- We construct dictionaries for both, giving us a term `typeRepArray# :: TypeRep (* -> #) Array#`

While nothing seems to blow up with this patch,

```patch
diff --git a/compiler/coreSyn/CoreLint.hs b/compiler/coreSyn/CoreLint.hs
index 99625c9..2c401de 100644
--- a/compiler/coreSyn/CoreLint.hs
+++ b/compiler/coreSyn/CoreLint.hs
@@ -1040,7 +1040,7 @@ lintType ty@(TyConApp tc tys)
   = lintType ty'   -- Expand type synonyms, so that we do not bogusly complain
                    --  about un-saturated type synonyms
 
-  | isUnliftedTyCon tc || isTypeSynonymTyCon tc || isTypeFamilyTyCon tc
+  | isTypeSynonymTyCon tc || isTypeFamilyTyCon tc
        -- Also type synonyms and type families
   , length tys < tyConArity tc
   = failWithL (hang (text "Un-saturated type application") 2 (ppr ty))
```

I otherwise have no reason to believe that this is safe.

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

| Trac field             | Value             |
| ---------------------- | ----------------- |
| Version                | 8.1               |
| Type                   | FeatureRequest    |
| TypeOfFailure          | OtherFailure      |
| Priority               | normal            |
| Resolution             | Unresolved        |
| Component              | Compiler          |
| Test case              |                   |
| Differential revisions |                   |
| BlockedBy              |                   |
| Related                |                   |
| Blocking               |                   |
| CC                     | goldfire, simonpj |
| Operating system       |                   |
| Architecture           |                   |

</details>