A small doubt

Patrik Jansson patrikj@cs.chalmers.se
Sat, 20 Oct 2001 17:48:30 +0200 (MET DST)

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]