The meaning of tilde in Haskell types (Type equality)

Question

I\'ve been messing around with the fix function, and I happened across this:

λ let fix f = let x = f x in x
λ fix (+)

:15:5:         


        

Answer 1

Tilde (~) in that error means type equality. It is telling you that it cannot deduce t to be t -> t. The symbol is also used for irrefutable patterns, but that's a completely different context.