[Haskell-cafe] If you'd design a Haskell-like language, what would you do different?

[Jesse Schalken]

On Tue, Dec 20, 2011 at 9:47 PM, Gregory Crosswhite
<gcrosswhite at gmail.com>wrote:

>
> On Dec 20, 2011, at 8:40 PM, Jesse Schalken wrote:
>
> If you think a value might not reduce, return an error in an error monad.
>
>
> Okay, I'm completely convinced!  Now all that we have to do is to solve
> the halting problem to make your solution work...  :-)
>

Why do you have to solve the halting problem?

Consider integer division by 0.

intDiv x y = if y > x then 0 else 1 + (intDiv (x - y) y)


This is correct, but does not reduce with y = 0. The Prelude version
returns bottom in this case. Here is a version that returns an error in an
Either String.

intDiv :: (Ord a, Num a) => a -> a -> Either String a

intDiv _ 0 = Left "Division by 0!"
intDiv x y = if y > x then Right 0 else intDiv (x - y) y >>= (Right . (1 +))


This is all I was talking about.



Cheers,
> Greg
>
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