What is XNoMonomorphismRestriction?

Question

This page usages

$ ghci -XNoMonomorphismRestriction

to start the haskell interpreter.

What does XNoMonomorphismRestriction switch mean?

Answer 1

It turns off the Monomorphism restriction, which restricts values which are not defined using "function notation"¹ to have a non-polymorphic type.

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¹ By "not using function notation" I mean that they're defined as foo = something and not foo bar = something, i.e. the definition does not contain explicit arguments.