[Haskell-cafe] Rebindable 'let' for observable sharing?
Tillmann Rendel
rendel at informatik.uni-marburg.de
Tue Mar 1 11:53:07 CET 2011
Hi,
Bas van Dijk wrote:
> For the record: are you talking about rewriting:
>
> let f = e in b
>
> into something like:
>
> (\f -> e) `letin` (\f -> b)
>
> where `letin` can be overloaded ("rebinded" is probably the better
> term) and has the default implementation:
>
> letin :: (a -> a) -> (a -> b) -> b
> fe `letin` fb = fb (fix fe)
I don't see how this captures the static semantics of let.
In the original code, f can be polymorphic, but in the desugared
version, f is monomorphic. Maybe the following works with an appropriate
set of extensions enabled:
First, infer the type of f in (let f = e in b) and call it T.
Then, rewrite (let f = e in b) to ((\f :: T -> e) `letin` (\f -> b)).
But Henning's original goal to "open a way to implement observable
sharing as needed in EDSLs by a custom 'fix'" might be easier to achieve
by desugaring let expressions into let expressions with some extra
function applications:
let f = e in b
would become something like
let f = hook1 e in hook2 f b
Now, hook1 would default to id, and hook2 would default to (const id). I
think this would capture the static and dynamic semantics of let
expressions, and it might be enough to implement observable sharing.
Tillmann
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