[Haskell-cafe] A probably-stupid question about a Prelude implementation.

[Jan-Willem Maessen]

On Jun 22, 2007, at 2:30 PM, Dan Weston wrote:

> This is how I think of it:
>
> lazyIntMult :: Int -> Int -> Int
> lazyIntMult 0 _ = 0
> lazyIntMult _ 0 = 0
> lazyIntMult a b = a * b
>
> *Q> 0 * (5 `div` 0)
> *** Exception: divide by zero
> *Q> 0 `lazyIntMult` (5 `div` 0)
> 0
>
> foldr evaluates a `f` (b `f` (c `f` ...))
>
> Only f knows which arguments are strict and in which order to  
> evaluate them. foldr knows nothing about evaluation order.

And, indeed, if you foldr a function with left zeroes, and you check  
for them explicitly as lazyIntMult and (||) do, then foldr is  
guaranteed to terminate early if it finds a zero.

z is a left zero of op if for all x, z `op` x = z.

This isn't the only time foldr will terminate early, but it is an  
important one.

-Jan-Willem Maessen

>
> Dan

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