Wredundant-constraints does not work when constraint synonym is used

{-# LANGUAGE ConstraintKinds #-}

{-# OPTIONS_GHC -Wredundant-constraints #-}

module Main where

import Data.List

main :: IO ()
main = return ()

withWarning :: (Show a, Ord a) => [a] -> [a]
withWarning = sort

type ConstraintSynonym a = (Show a, Ord a)

withoutWarning :: ConstraintSynonym a => [a] -> [a]
withoutWarning = sort

Main.hs:12:1: warning: [-Wredundant-constraints]
    • Redundant constraint: Show a
    • In the type signature for:
           withWarning :: forall a. (Show a, Ord a) => [a] -> [a]
   |
14 | withWarning :: (Show a, Ord a) => [a] -> [a]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The withoutWarning function does not need the (Show a) constraint but there is no warning for it.

Trac metadata

Trac field	Value
Version	8.4.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed milestone to %8.6.1

changed weight to 5

added Tbug Trac import labels

Yes, this is a little awkward

• withWarning has two separate constraints; its type is really curried, like this:

withWarning :: Show a => Ord a => [a] -> [a]

• withoutWarnig only has one constraint, a tuple. It is uncurried. This is so even though it's only a synonym, so that if you ask say :t withoutWarning it'll display the type with the synonym.

But the result is that the compound constraint for withoutWarning is in fact used (by sort).

I'm sure it'd be possible to fix this, but there's a bit of a grey zone. What about

type family F a :: Constraint
type instance F [a] = (Show a, Ord a)

foo :: F [a] => [a] -> [a]

Do you expect a warning for that? A type system is a degenerate form of this.

Simon, i don't know about type families. I tried reading about them at https://wiki.haskell.org/GHC/Type_families but even so i don't feel i have enough experience with them to give a useful response to your question. It's also more confusing for me because in your example foo doesn't have an implementation. With withoutWarning = sort it was clear to me that sort requires Ord but not Show.

I will make a guess here and say "yes i expect a warning for that". When the instance F [a] is used in used in the function foo it can be deduced that this is the is the active instance which is used in the function foo. If foo is implemented as foo = sort then (Show a) is also redundant in the instance of the .. constraint type family ?

My guess can be non-sense .. please refer to the first paragraph :P

changed milestone to %8.8.1

These won't be fixed for in GHC 8.6.

changed milestone to %8.10.1

Bumping milestones of low-priority tickets.

added Pnormal label

changed milestone to %9.0.1

changed milestone to %9.2.1

This will not be fixed in time for %9.2.1

changed milestone to %9.4.1

Closing as I am not sure this is a bug in the first place.

removed milestone %9.4.1

closed