[Haskell-cafe] Re: If wishes were horses...
wren ng thornton
wren at freegeek.org
Mon Mar 15 23:58:12 EDT 2010
Ben Millwood wrote:
> In general, laziness behaviour can
> get complicated quickly and so I'm not convinced that the type
> signature is a good home for that information.
Certainly it can. A lot of the same problems arise in the logic
programming community under the topic of "modes", i.e. whether a logic
variable must be ground (or rather, how defined it must be) before it's
safe to "run the function backwards". Though, at present AFAIK, they've
contented themselves with rough approximations like lifted/unlifted
value types or strict/lazy arrows, either of which can catch the simple
Personally, I think the only way to capture *all* strictness information
is if we have full dependent types (or worse, for logic languages, since
their dependencies lack the directionality of usual dependent types). If
we had full dependent types, and went with the lifted/unlifted
distinction instead of the strict/lazy one, then we could give `maybe`
maybe :: (_:b) -> (_:a -> b') -> (m:Maybe a)
-> (case m of Nothing => b ; Just _ => b')
Would such a type be helpful? <shrug> I think adding full dependent
types is a bit much if all we're interested in is strictness behavior.
But, as you say, it seems very unlikely that we can encode strictness
behaviors which may depend on particular runtime values without DTs.
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