Why are polymorphic values not inferred in Haskell?

Question

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

Answer 1

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