[Haskell-cafe] Rebindable 'let' for observable sharing?

Tillmann Rendel rendel at informatik.uni-marburg.de
Tue Mar 1 11:53:07 CET 2011


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.


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