GHC 7.10 regression when using foldr

20 Jan 2015

      I just discovered that the following program compiled fine using GHC
7.8.4 but was rejected by GHC 7.10.1-rc1:

~~~
data List a = Nil | Cons a (List a)

instance Read a => Read (List a) where
  readsPrec d s = map convert (readsPrec d s)
    where
    convert (xs, s2) = (foldr Cons Nil xs, s2)
~~~

GHC 7.10 now complains:

~~~
Read.hs:5:23:
    Could not deduce (Foldable t0) arising from a use of ‘convert’
    from the context (Read a)
      bound by the instance declaration at Read.hs:4:10-32
    The type variable ‘t0’ is ambiguous
    Note: there are several potential instances:
      instance Foldable (Either a) -- Defined in ‘Data.Foldable’
      instance Foldable Data.Proxy.Proxy -- Defined in ‘Data.Foldable’
      instance GHC.Arr.Ix i => Foldable (GHC.Arr.Array i)
        -- Defined in ‘Data.Foldable’
      ...plus three others
    In the first argument of ‘map’, namely ‘convert’
    In the expression: map convert (readsPrec d s)
    In an equation for ‘readsPrec’:
        readsPrec d s
          = map convert (readsPrec d s)
          where
              convert (xs, s2) = (foldr Cons Nil xs, s2)

Read.hs:5:32:
    Could not deduce (Read (t0 a)) arising from a use of ‘readsPrec’
    from the context (Read a)
      bound by the instance declaration at Read.hs:4:10-32
    The type variable ‘t0’ is ambiguous
    Relevant bindings include
      readsPrec :: Int -> ReadS (List a) (bound at Read.hs:5:3)
    Note: there are several potential instances:
      instance (Read a, Read b) => Read (Either a b)
        -- Defined in ‘Data.Either’
      instance forall (k :: BOX) (s :: k). Read (Data.Proxy.Proxy s)
        -- Defined in ‘Data.Proxy’
      instance (GHC.Arr.Ix a, Read a, Read b) => Read (GHC.Arr.Array a b)
        -- Defined in ‘GHC.Read’
      ...plus 18 others
    In the second argument of ‘map’, namely ‘(readsPrec d s)’
    In the expression: map convert (readsPrec d s)
    In an equation for ‘readsPrec’:
        readsPrec d s
          = map convert (readsPrec d s)
          where
              convert (xs, s2) = (foldr Cons Nil xs, s2)
~~~

The reason is the usage of foldr, which changed its type from

  foldr :: (a -> b -> b) -> b -> [a] -> b -- GHC 7.8.4

to

  foldr :: Foldable t => (a -> b -> b) -> b -> t a -> b -- GHC 7.10.1

Thus, the use of foldr is now ambiguous. I can fix this by providing a
type signature

  convert :: ([a], String) -> (List a, String)

However, is this breaking change intended?

Regards,
Björn

GHC 7.10 regression when using foldr

Björn Peemöller