On 13 Oct 2004, at 06:59, Serge D. Mechveliani wrote:
bottomStr :: String bottomStr = let xs = xs in xs
bottomSPair :: (String, String) bottomSPair = let p = p in p
Note here that bottomSPair is bottom...
addToSPair :: Char -> (String, String) -> (String, String) addToSPair c (xs, ys ) = (c:xs, ys )
In a more clear presentation, the computation should (?) be
head $ fst $ addToSPair 'a' bottomSPair = head $ fst $ addToSPair 'a' (bottomStr, bottomStr)
...so this step is wrong: bottomSPair is not the same as the pair (bottomStr, bottomStr). Haskell pairs are lifted: (bottom,bottom) is different from bottom.
bottomSPair is of type (String, String).
addToSPair is declared as returning (String, String).
Hence, the complier knows ab initio that after addToSPair
the result is of kind
('a': _, _) :: (String, String)
No. The result is necessarily of type (String,String), but need not be a proper pair. (And indeed, addToSPair c bottom yields bottom, not a proper pair.)
So, similarly as head ('a' : bottomStr) = 'a', (1) it should be head $ fst $ addToSPair 'a' bottomSPair = 'a' (2)
Question 1 ---------- What may be the consequences for the language (let us call it HCLazy) if it treats the data constructors like in (2), like more `lazy' ones?
That is, what if Haskell had unlifted pairs? Interesting question. Nils Anders Danielsson (http://www.cs.chalmers.se/~nad/) has been looking at it, with his work on Chasing Bottoms (http://www.cs.chalmers.se/~nad/software/ChasingBottoms/docs/).
For example, fst (bottom :: (a,b)) = (bottom :: a)
I don't think this example is what you mean. This applies in Haskell already. I think what you mean is that (fst p, snd p) equals p for every p, including p=bottom.
How to arrange the above `lazy' output? The whole result story is ready in several hours, or days, and each step should be displayed immediately as it is ready.
Another poster mentioned the possibility of using an irrefutable pattern (by adding a tilde). Equivalently, you can define
addToSPair :: Char -> (String, String) -> (String, String) addToSPair c p = (c : fst p, snd p)
Then addToSPair c bottom is (c:bottom,bottom) as you're hoping, not bottom.
Jeremy
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