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  • Glasgow Haskell Compiler
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  • #3584
Closed
Open
Created Oct 14, 2009 by baramoglo@trac-baramoglo

type signature involving a type family rejected

add1 is rejected in the following program:

{-# LANGUAGE EmptyDataDecls, FlexibleContexts, FlexibleInstances, GADTs, TypeFamilies, TypeOperators, UndecidableInstances #-}

data False

type family IsNegative x

type family x :+: y

class Natural x
instance (IsNegative x ~ False) => Natural x

data A n
  where
    A :: Natural n => Int -> A n

getA :: A n -> Int
getA (A n) = n

add1 :: Natural (m:+:n) => A m -> A n -> A (m:+:n)
add1 (A a) (A b) = A (a+b)

add2 (A a) (A b) = A (a+b)

add3 :: (IsNegative (m:+:n) ~ False) => A m -> A n -> A (m:+:n)
add3 (A a) (A b) = A (a+b)

add4 :: Natural (m:+:n) => A m -> A n -> A (m:+:n)
add4 a b = A (getA a + getA b)

add5 :: Natural (m:+:n) => A m -> A n -> A (m:+:n)
add5 (A a) _ = A a

The following modifications of add1 work fine:

  • Removing the type signature (add2)
  • Using the type inferred for add2 (add3)
  • Using the projection function getA instead of pattern matching (add4)
  • Only pattern match on the first argument
Trac metadata
Trac field Value
Version 6.10.2
Type Bug
TypeOfFailure OtherFailure
Priority normal
Resolution Unresolved
Component Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC 78emil@gmail.com
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