learning to love laziness

[Mark Tullsen]

Haskell has lazy/lifted products and not true products.  This "feature"
is considered by many to be an unfortunate aspect of Haskell.  A 2-tuple
is just syntactic sugar for
   data Tuple2 a b = Tuple2 a b

Maybe from seeing this, it's clearer why laws such as
   x = (fst x,snd x)
do not hold.  Neither does the following law hold
    (uncurry . curry) f = f
which is unfortunate (for a language named after Haskell *Curry*).
To see why it doesn't hold, compare t1 and t2 in this program:
   f (_,_) = 1
   t1 = f undefined
   t2 = (uncurry . curry) f undefined

- Mark

On Wednesday, September 24, 2003, at 02:07  PM, Norman Ramsey wrote:
> Consider the following Haskell function:
>
>> asPair x = (fst x, snd x)
>
> This function has type forall a b. (a, b) -> (a, b)
> and is almost equivalent to the identity function, except it
> can be used to make programs terminate that might otherwise fall
> into a black hole.
>
> My students are extremely mystified by such functions---and I can 
> hardly
> blame them!  Is there a good place to read about programming with lazy
> evaluation that will cover such functions and when to use them?
>
>
> Norman
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