[Haskell-cafe] what does the '~' mean ?

[Ryan Ingram]

On Thu, Apr 15, 2010 at 10:59 PM, zaxis <z_axis at 163.com> wrote:
>
> instance (BinaryDefer a, BinaryDefer b) => BinaryDefer (a,b) where
>    put (a,b) = put2 a b
>    get = get2 (,)
>    size x = let ~(a,b) = x in size a + size b
>    putFixed (a,b) = putFixed2 a b
>    getFixed = getFixed2 (,)
>
> in `size` function, what does the `~` mean ?

This is kind of a funny question, because in this case the ~ doesn't
mean anything at all.  Pattern matches in let are automatically
irrefutable/lazy.

A better way to write this is

size ~(a,b) = size a + size b

which is equivalent to

size x = size a + size b where
   (a,b) = x

which is equivalent to

size x = let (a,b) = x in size a + size b

which is equivalent to

size x = let
    a = case x of (v,_) -> v
    b = case x of (_,v) -> v
  in size a + size b

If a or b never get evaluated, the case statements (which will fail on
bottom values) don't happen.