id,summary,reporter,owner,description,type,status,priority,milestone,component,version,resolution,keywords,cc,os,architecture,failure,testcase,blockedby,blocking,related,differential,wikipage
1807,type equality coercions not symmetric + order dependent,guest,chak,"I'm not entirely sure this is a bug, but it seems pretty weird behaviour:
`a ~ b` cannot always be replaced by `b ~ a`
`a ~ b, c ~ d` cannot always be replaced by `c ~ d, a ~ b`
{{{
{-# LANGUAGE TypeFamilies, GADTs #-}
module Test where
data List n where
-- Works
--Cons :: (Maybe n ~ n') => List n -> List n'
--Cons :: (Maybe n ~ n', n' ~ Maybe n) => List n -> List n'
-- Fails
Cons :: (n' ~ Maybe n) => List n -> List n'
--Cons :: (n' ~ Maybe n, Maybe n ~ n') => List n -> List n'
class Snoc i where
snoc :: List i -> List (Maybe i)
instance Snoc i => Snoc (Maybe i) where
snoc (Cons xs) = Cons (snoc xs)
}}}
This module compiles fine with both 6.8.0.20071025 and 6.9.20071025 using one of the first two Constructors. With either of the last two constructors both versions say:
{{{
Could not deduce (Snoc n) from the context ()
arising from a use of `snoc' at Test.hs:17:27-33
Possible fix: add (Snoc n) to the context of the constructor `Cons'
In the first argument of `Cons', namely `(snoc xs)'
In the expression: Cons (snoc xs)
In the definition of `snoc': snoc (Cons xs) = Cons (snoc xs)
}}}",bug,closed,normal,6.10 branch,Compiler (Type checker),6.8,duplicate,type equality coercions symmetry order constraints,rturk@…,Unknown/Multiple,Unknown/Multiple,,,,,,,