Re: [Haskell-cafe] Is "let" special?

When you use arrows for your DSL, how do you avoid getting trapped by "arr"? It seems like it's hard to avoid people working arbitrary functions into your computation using it. -- ryan On Thu, Nov 4, 2010 at 2:46 PM, Henning Thielemann wrote:

Max Bolingbroke schrieb:

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2010/11/2 Günther Schmidt :

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I've been given a few hints over time when I asked question concerning DSLs but regretfully didn't follow them up.

I think this is probably to do with observable sharing. The problem in DSLs is if you have:

fact :: Term -> Term fact = Factorial

instance Num Term where   fromIntegral = Int   (+) = Plus

e = let x = fact 2 :: Term     in x + x :: Term

(Lets say the terms of our deeply embedded DSL are of type Term). If you pattern match on "e" you will get something like (Factorial (Int 2)) `Plus` (Factorial (Int 2)). This has lost the work sharing implicit in the original term, where the Factorial computation was shared.

To recover this sharing, you either need some sort of observable sharing, or some common subexpression elimination (which risks introducing space leaks if your DSL has lazy semantics). Quite a lot of papers have mentioned this issue: the one that springs to mind is Gill's "Type Safe Observable Sharing".

Actually, I often need some replacement for 'let' in EDSL's - a sharing 'let' on the level of the target language. So far I have solved such issues by Arrows (fact :: EDSLArrow Term Term). Others have solved it by searching and sharing common sub-expressions.

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