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 _______________________________________________ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
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