~ patterns

Simon Marlow simonmar at microsoft.com
Tue Jan 31 18:05:37 EST 2006

On 30 January 2006 12:32, John Hughes wrote:

> I don't really agree that ~ adds complexity to the language design,
> given that we need
> SOME mechanism for lazy pattern matching. If you write
> a denotational semantics of pattern matching, then ~ fits in
> naturally. It has a simple,
> compositional semantics. Where I see complexity is that a pattern
> means different
> things in a let or in a case, so I have to remember, for each
> construct, whether it's
> a strict-matching or lazy-matching construct. Quick, when I write do
> (x,y)<-e...
> is that pair matched strictly, like case, or lazily, like let? If
> strictly, then why? There's
> no choice to be made here, so why not wait until x or y is used before
> matching? I expect
> you know the answer to this question off-hand, but it's an obstacle to
> learning the
> language--I'll bet there are many quite experienced Haskell
> programmers who are
> uncertain. If only pattern matching was *always* strict, unless a ~
> appeared, then the
> language would be more regular and easier to learn.

For what it's worth, I agree with this point.  I'd be quite happy for
pattern matching to be strict by default in let and where.

I can imagine it might still be confusing to some, though, because

   let x = fac 99

does not evaluate 'fac 99', but

   let (x,y) = quotRem a b

does evaluate 'quotRem a b'.  Still, I suppose it's no more confusing
than the current situation.

If pattern matching in where was strict, I imagine I'd use ~ a lot more.
A common practice is to throw a bunch of bindings into a where clause,
with no regard for whether they get evaluated or not - variable bindings
and pattern bindings alike.  If the pattern bindings are strict, I have
to add ~ to each one to get the same effect.

On the other hand, if pattern bindings were strict by default, I bet
there would be a lot fewer accidental space leaks.


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