Numeric literals have a polymorphic type:
*Main> :t 3
3 :: (Num t) => t
But if I bind a variable to such a literal, the polymorphism is lost:
x = 3
...
*Main> :t x
x :: Integer
If I define a function, on the other hand, it is of course polymorphic:
f x = 3
...
*Main> :t f
f :: (Num t1) => t -> t1
I could provide a type signature to ensure the x remains polymorphic:
x :: Num a => a
x = 3
...
*Main> :t x
x :: (Num a) => a
But why is this necessary? Why isn't the polymorphic type inferred?
It's the monomorphism restriction which says that all values, which are defined without parameters and don't have an explicit type annotation, should have a monomorphic type. This restriction can be disabled in ghc and ghci using -XNoMonomorphismRestriction.
The reason for the restriction is that without this restriction long_calculation 42 would be evaluated twice, while most people would probably expect/want it to only be evaluated once:
longCalculation :: Num a => a -> a
longCalculation = ...
x = longCalculation 42
main = print $ x + x