On Sat, 20 Oct 2001, Adrian Hey wrote: ... : Interesting. I always thought pattern matching was lazy, but it I don't think Haskell has changed here (recently).
main = print test3
This does not work:
test1= let [a,b,c] = "Hello" in [a,b]
Same here:
test2= let a: b: c: [] = "Hello" in [a,b]
But this works - thanks to the irrefutable pattern.
test3= let a: b: ~(c: []) = "Hello" in [a,b]
This also works as variables (here cs) are irrefutable.
test4= let a: b: cs = "Hello" c: [] = cs in [a,b]
irrefutable = will always (pretend to) match (may fail later) = "lazy" match (don't match unless you have to) = delayed match (delayed to first use of some variable in the pattern) On Sat, 20 Oct 2001, Adrian Hey wrote: : Putting a ~ in front of the pattern [a,b,c] makes no difference. But the The "marker" ~ for irrefutable patterns only work "one level" down, so in ~[a,b,c] only the outermost (:) is affected. But in let the outermost pattern is lazy anyway so there is no difference. Further examples: This also works: (the (mis)match with the empty list is delayed until the end of time)
test5= let a: b: ~(c: ~[]) = "Hello" in [a,b,c]
But this fails (if the full list is requested ...)
test6= let a: b: ~(c: []) = "Hello" in [a,b,c]
/Patrik