Opened 16 months ago

Closed 15 months ago

## #11667 closed bug (fixed)

# Incorrect pattern synonym types in error messages

Reported by: | rdragon | Owned by: | rdragon |
---|---|---|---|

Priority: | normal | Milestone: | 8.0.1 |

Component: | Compiler | Version: | 8.0.1-rc2 |

Keywords: | PatternSynonyms | Cc: | simonpj, mpickering |

Operating System: | Unknown/Multiple | Architecture: | Unknown/Multiple |

Type of failure: | Incorrect warning at compile-time | Test Case: | |

Blocked By: | Blocking: | ||

Related Tickets: | #11213 | Differential Rev(s): | Phab:D1967 |

Wiki Page: |

### Description

Here are some examples of incorrect pattern synonym types in error messages.

**Example 1**

{-# LANGUAGE PatternSynonyms #-} module Mod1 where pattern Pat :: Eq a => Maybe a pattern Pat <- Just 42

This gives the error (note the missing `Eq a =>`

):

Mod1.hs:4:21: error: * Could not deduce (Num a) arising from the literal `42' from the context: Eq a bound by the type signature for pattern synonym `Pat': Maybe a at Mod1.hs:4:9-11 Possible fix: add (Num a) to the context of the type signature for pattern synonym `Pat': Maybe a * In the pattern: 42 In the pattern: Just 42 In the declaration for pattern synonym `Pat'

**Example 2**

{-# LANGUAGE PatternSynonyms, GADTs #-} module Mod1 where data T where MkT :: a -> T pattern Pat :: () => b ~ Bool => a -> b -> (b, T) pattern Pat x y = (y, MkT x)

We get the error (note the missing contexts and the incorrect quantification `forall b`

, missing `a`

):

Mod1.hs:6:27: error: * Couldn't match type `b' with `Bool' arising from the "provided" constraints claimed by the signature of `Pat' `b' is a rigid type variable bound by the type signature for pattern synonym `Pat': forall b. a -> b -> (b, T) at Mod1.hs:5:16 * In the declaration for pattern synonym `Pat' * Relevant bindings include y :: b (bound at Mod1.hs:6:20)

**Example 3**

{-# LANGUAGE PatternSynonyms #-} module Mod1 where pattern Pat :: Eq a => Show a => a -> Maybe a pattern Pat x <- Just x

We get the error:

Mod1.hs:4:23: error: * Could not deduce (Show a) arising from the "provided" constraints claimed by the signature of `Pat' from the context: Eq a bound by the type signature for pattern synonym `Pat': a -> Maybe a at Mod1.hs:4:9-11 * In the declaration for pattern synonym `Pat'

Here we could perhaps remove the whole "from the context ... at Mod1.hs:4:9-11" part, as it doesn't make much sense to tell the user that we could not deduce a "provided" constraint from the "required" context.

**Example 4**

{-# LANGUAGE PatternSynonyms, GADTs #-} module Mod1 where data T a where MkT :: (Num a, Show a) => a -> T a pattern Pat :: (Eq a) => (Show a) => T a pattern Pat = MkT 42

This gives the error (note that the signature in the error is of the pattern synonym `Pat`

when used in an expression context):

Mod1.hs:6:15: error: * Could not deduce (Num a) arising from a use of `MkT' from the context: (Eq a, Show a) bound by the type signature for pattern synonym `Pat': (Eq a, Show a) => T a at Mod1.hs:6:1-20 Possible fix: add (Num a) to the context of the type signature for pattern synonym `Pat': (Eq a, Show a) => T a * In the expression: MkT 42 In an equation for `$bPat': $bPat = MkT 42

The problems in these four examples are caused by the use of `SigSkol (PatSynCtxt n) (Check ty)`

for representing pattern synonym types (and the types of bidirectional pattern synonyms when used in an expression context).

A possible solution can be found in Phab:D1967.

Simon notes the following:

Example 3 raises an interesting point we can debate once we have a ticket. Namely, is this ok?

pattern Pat :: Ix a => Ix a => a -> Maybe a pattern Pat x <- Just x

### Change History (5)

### comment:1 Changed 16 months ago by

Milestone: | → 8.0.2 |
---|

### comment:3 Changed 15 months ago by

Milestone: | 8.0.2 → 8.0.1 |
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Status: | new → merge |

### comment:5 Changed 15 months ago by

Resolution: | → fixed |
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Status: | merge → closed |

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There is a detailed comment describing the errors before and after rdragon's proposed solution on Phab:D1967.