Understanding how the pure function is resolved in Haskell

Question

In GHCi when I type pure 2 it returns 2; or pure "aa" returns "aa". I wonder how this applicative instance is resolved for 2 or "aa" by GHCi.

thank you @user3608068. The link you provided exactly answers my question but I was not able to find it when I searched for it. — milad zahedi, Dec 31 '16 at 06:01

score 6 · Accepted Answer · answered Dec 29 '16 at 13:01

GHCi performs some magic to be user-friendly.

When entering an expression whose type is of the form ... => f a, it tries to instantiate f to IO. In your case, this is possible since IO is an applicative (and a monad).

Secondly, when an expression having a type of the form ... => IO a is entered, it is run as an IO action.

Finally, if a is of class Show, the result is printed. In your case "aa" is the result (and the type a is String), so GHCi prints that.

Thank you Mr chi, I feel enlightened now :) – milad zahedi Dec 31 '16 at 05:58 — milad zahedi, Dec 31 '16 at 05:58

Understanding how the pure function is resolved in Haskell

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