Questions tagged [monomorphism-restriction]

Haskell's monomorphism restriction causes some functions that should be polymorphic to be forced into monomorphism, causing errors. For instance:

sum = foldr (+) 0

in GHC 7.10, this will fail with this message:

No instance for (Foldable t0) arising from a use of ‘foldr’
The type variable ‘t0’ is ambiguous
Relevant bindings include
  mysum :: t0 Integer -> Integer (bound at src/Main.hs:37:1)
Note: there are several potential instances:
  instance Foldable (Either a) -- Defined in ‘Data.Foldable’
  instance Foldable Data.Functor.Identity.Identity -- Defined in ‘Data.Functor.Identity’
  instance Foldable Data.Proxy.Proxy -- Defined in ‘Data.Foldable’
  ...plus five others
In the expression: foldr (+) 0
In an equation for ‘mysum’: mysum = foldr (+) 0

This can usually be solved by explicitly defining the type signature:

sum :: (Foldable f, Num a) => f a -> a

Or by removing the restriction:

{-# LANGUAGE NoMonomorphismRestriction #-}