haskell - Monomorphism Restriction, ghci, and let expression

Question

This expression is incorrect.

f = show

However, in ghci this is legit

let f = show

Moreover, its type is changed to

() -> String

Is there any explanation of this phenomenon?

Answer 1

The ghci prompt behaves as if the ExtendedDefaultRules extension is enabled.

In particular this means that:

The unit type () is added to the start of the standard list of types which are tried when doing type defaulting.

So to get the same behaviour from a source file, either compile with -XExtendedDefaultRules, or add {-# LANGUAGE ExtendedDefaultRules #-} to the top of the file.