Serge D. Mechveliani wrote:
As the types are resolved before the computation, as the above program shows, how can addToSPair expect anything besides a string pair? Why tilda is not a default?
Haskell pairs are "lifted", meaning that they also include an extra value (bottom) which doesn't match (x,y). Not everyone is convinced that this is a good idea, but it's the way things are at present.
The only doubt may be the matching of (xs, ys) against bottom :: (String, String) Is not this natural to have a result as (bottom :: String, bottom :: String) -- = xs = ys and compute further?
Possibly in this case, yes. But not in general, since there might be other data constructors also.
What will occur if all the Haskell programmers set `~' before each data constructor in each pattern in their existing programs?
Things won't work as you'd expect. For example, if you defined null :: [a] -> Bool null list = case list of ~(x:xs) -> False ~[] -> True you would find that (null []) is False, not True, because the first pattern matches irrefutably. Pattern matching needs to be strict so that case statements like this work sensibly.
As the types are recognized before the computation, the programs will remain safe and will become considerably faster. For example, the above program of g1 becomes a billion times faster. I wonder.
Actually using irrefutable patterns tends to make a program slower, not faster, because it makes the program less strict. Good questions! -- Ben