Chris Eidhof wrote:
When binding the function composition to a variable, the type suddenly changes.
Prelude Control.Arrow List> :t map (length &&& head) . group map (length &&& head) . group :: (Eq a) => [a] -> [(Int, a)] Prelude Control.Arrow List> let encode = map (length &&& head) . group Prelude Control.Arrow List> :t encode encode :: [Integer] -> [(Int, Integer)]
You've tripped over the Monomorphism Restriction. http://haskell.org/haskellwiki/Monomorphism_restriction http://haskell.org/onlinereport/decls.html#sect4.5.5 In short, you have to supply a type signature encode :: (Eq a) => [a] -> [(Int, a)] encode = map (length &&& head) . group to get the polymorphic function type when type-classes like Eq or especially Num are involved. Without signature, the compiler will _default_ some the type variables mentioned in the class context to Integer or similar. Note that definitions on the GHCi prompt will receive more defaulting than those in Haskell source files. This is to make things like show [] 1+5 work at the prompt. Also note that the monomorphism restriction only applies to constant applicative forms, i.e. point-free definitions of values. In other words, encode x = map (length &&& head) . group $ x will result in the proper polymorphic type. Regards, apfelmus