There’s another factor here, mentioned in some of the links which acfoltzer includes, but it might be worth making explicit here. You’re encountering the effect of the monomorphism restriction. When you say let x = 5 you make a top-level definition of a variable. The MR insists that such definitions, when otherwise unaccompanied by a … Read more
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 … Read more
The gist is that the with the polymorphic xs it has a type of the form xs :: Num a => [a] typeclasses under the hood are really just functions, they take an extra argument that GHC automatically fills that contains a record of the typeclasses functions. So you can think of xs having the … Read more
What is the monomorphism restriction? The monomorphism restriction as stated by the Haskell wiki is: a counter-intuitive rule in Haskell type inference. If you forget to provide a type signature, sometimes this rule will fill the free type variables with specific types using “type defaulting” rules. What this means is that, in some circumstances, if … Read more