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.
Thanks,
David
On 19 June 2017 at 00:49, Francesco Ariis
On Sun, Jun 18, 2017 at 05:02:13PM +0800, Choi Waikit wrote:
Hi guys,
I have difficulty understanding how GHC determines what kind of error message to return. Could you help explain a bit ? Thanks !
The code can be accessed here http://lpaste.net/356325
My question is Why does calling `settleDown 9` return different error message from `settleDown "sting"` ?
Hello David, every time you write a number in a haskell source, it gets automagically wrapped in `fromInteger` (actually, `fromInteger` if there isn't a dot, `fromRational` otherwise). This is made to provide some kind of overloading.
`fromInteger` has signature:
λ> :t fromInteger fromInteger :: Num a => Integer -> a
hence the different error with 9 and "string"!
Is it clear now? _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners