On Mon, Jun 19, 2017 at 10:56:59AM +0800, David CAI wrote:
Hi Francesco,
Thanks for your reply! Just to make sure I understood, can I paraphrase your words as the following ?
In the case of `settleDown 9` `settleDown 9` is first translated to `settleDown $ fromInteger 9`. Since fromInteger lies in Num type class and settleDown's signature is: settleDown :: Mood -> Mood, the compiler tries to look for an instance for Num typeclass so that it can resolve which `fromInteger` method to call. Therefore it doesn't show a type mismatch error but hint for implementing a typeclass.
In the case of `settleDown "hello"` There's no preprocessing for "hello" so its type remains [Char]. Since settleDown has type Mood -> Mood, Mood and [Char] are not compatible, therefore the compiler shows a type mismatch error.
Excellent explanation, that's exactly it!