[Haskell-beginners] How to create a monad in GHC 7.10 or newer

[Francesco Ariis]

Hello Ahmad,

Il 13 novembre 2022 alle 16:33 Ahmad Ismail ha scritto:
> Due to lack of examples, I am not understanding how to implement >>= and
> >>.

All you need to implement is (>>=)!

> The code I came up with so far is:
> 
> instance Monad (WhoCares a) where
>     (>>=) :: Matter a -> (a -> Matter b) -> Matter b
>     (>>) :: Matter a -> Matter b -> Matter b
>     return :: a -> Matter a
>     return = pure

The signature for (>>=) is wrong, `Matter` is a *data* constructor, you
need a *type* one instead, so:

    (>>=) :: WhoCares a -> (a -> WhoCares b) -> WhoCares b

But let us go back to typeclasses. Your `Applicative` instance

> instance Applicative WhoCares where
>   pure = Matter
>   Matter f <*> Matter a = Matter (f a)

is broken:

    λ> ItDoesnt <*> WhatThisIsCalled
    *** Exception: /tmp/prova.hs:11:5-40: Non-exhaustive patterns in function <*>

So we need first to fix that. What behaviour would you expect, what are
you trying to model with `WhoCares`?
—F