Category theory books are almost certainly not what you want to be looking
at. Yes, some things in Haskell are inspired by it; but they're entirely
usable wthout, and they're all rather simplified compared to the theory.
Typeclasses, you might want to start with the Typeclassopedia
https://wiki.haskell.org/Typeclassopedia. And understand that it is not
general function overloading, and you can get yourself into trouble by
trying to treat them as such: types flow "backwards" in Haskell, compared
to languages where overloading is common. If all you know about a type is
its name, you can't do anything with it. In most OO languages with
overloading, you can do anything you want with it and it'll throw an
exception if it doesn't support it; in Haskell, the compiler won't let you
get away with it at all, it never reaches the point of a runtime exception.
Similarly, if you know (Num a), this doesn't mean you can use division; you
need to also know (Integral a) to get div, or (Fractional a) to get (/).
One thing that follows from how this interacts with typeclasses is return
type polymorphism. Consider that maxBound takes no parameters, and decides
what to do based on the type it's used at.
On Thu, Jan 10, 2019 at 2:38 PM Damien Mattei
i'm not so pessimist,i'm beginning to have fun with haskell...really! about teaching ressource i will be happy to know where i can find them... i only post in haskell cafe when i have not find the answer online (tutorials,stackoverflow,real haskell book http://book.realworldhaskell.org/etc,etc... in the hunded pages i search i even get a look at Categories for the working Mathematician https://books.google.fr/books?id=6KPSBwAAQBAJ&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false) it is only when i have exhausted all the online ressource that i post to the cafe...
On Thu, Jan 10, 2019 at 6:52 PM Tom Ellis < tom-lists-haskell-cafe-2017@jaguarpaw.co.uk> wrote:
On Thu, Jan 10, 2019 at 04:23:49PM +0100, Damien Mattei wrote:
Le 10/01/2019 15:27, Tom Ellis a écrit :
On Thu, Jan 10, 2019 at 12:34:04PM +0100, Damien Mattei wrote:
i have this definition:
{-# LANGUAGE FlexibleInstances #-}
class ConcatenateMaybeString a where cms :: Maybe String -> a -> Maybe String
instance ConcatenateMaybeString (Maybe String) where cms mf ms = mf >>= (\f -> ms >>= (\s -> return (f ++ s)))
instance ConcatenateMaybeString String where cms mf s = mf >>= (\f -> return (f ++ s))
Trying to simulate overloading like this is ultimately going to lead to more frustration than benefit. I strongly suggest you just define two different functions.
those functions could be seen as a "style exercise" , for me,coming from untyped languages such as Scheme or LisP it's Haskell which is a frustration :-)
I think you're going to get significantly more frustrated with Haskell if you try to learn it like this by yourself rather than by working through some widely approved teaching resource. Of course, how you spend your time is up to you, but if you're frustrated with Haskell then trying to make it up as you go along is only going to worsen the feeling! _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.
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-- brandon s allbery kf8nh allbery.b@gmail.com