Type of return in do block

Question

I am trying to understand Monads in Haskell and during my countless experiments with code I have encountered this thing:

f2 = do   
  return \"da\"

Answer 1

This is known Monomorphism_restriction

Use signatures

f2 :: Monad m => m String
f2 = do   
  return "da"

or use language extension:

{-# LANGUAGE NoMonomorphismRestriction #-}
f2 = do   
  return "da"

to get valid code