Hello Simon, Monday, March 20, 2006, 1:47:52 PM, you wrote:
>> i've proposed to allow adding strict mark to any type constructors and >> type constructor parameters so that finally we can define any data >> structure that can be defined in strict languages. in particular: >> >> type StrictPair a b = !(,) a b >> type StrictElements a b = (,) !a !b >> type StrictBoth a b = !(,) !a !b >> type StrictFunction a b = !(->) !a !b >> >> strictMap :: StrictFunction a b -> ![!a] -> ![!b] >> >> where ![!a] is a strict list with strict elements SM> Bulat, this doesn't constitute a proposal. It leaves too many questions SM> unanswered. If it is supposed to be just syntactic sugar, and I believe SM> that is your intention, then can you show me how the above definitions SM> translate into Haskell 98? i'm not sure that i can make complete proposal, but i can say what i mean in more details: one of the differences between Haskell and most other languages is what even when we don't need laziness we are forced to "buy" it. so i want to see the language where laziness is optional at any place. shebang patterns allow to specify that concrete IMPLEMENTATION of some function is strict in its using of parameters. but this can't help us if we want to carry strict function in data structure, pass it as function argument, has is as a class member. i was bitten by this when i wrote Streams library - although Char encoding transformers are simple strict computations that just read several bytes and then return one Char, and byte reading operation by itself is very fast - they cannot be combined to fast Char-reading function. another problem is what while we can specify strictness of fields in ADTs, we cannot "redefine" strictness of fields in existing ADT, such as list. my solutions to these problems: 1) make a strictness annotation part of function type declaration, i.e. when function type can include strictness annotation on each of its arguments and on result: fac :: !Int -> !Int -> !Int strictness annotation on argument means that function is strict in this argument - if its value diverges then entire function diverges. informally, strict argument can be evaluated before evaluation of function body, as in the strict languages - what opens up possibility to unbox such values and to omit checking of argument evaluation in function body, moving this evaluation to the caller side strictness annotation on result means that function DON'T DIVERGE if all arguments are don't diverge. this allows to unbox result and to skip checking that result was evaluated on callee side by moving real computation inside the function. informally, this means that a function is inexpensive enough and therefore can be computed non-lazily 2) to allow "changing" of strictness inside existing ADTs, i propose to "copy" strictness annotations on type arguments to the type declaration bodies: data List a = Nil | Cons (List a) a type StrictElements a = List !a is equal to the: data StrictElements a = Nil | Cons (List a) !a i.e. it's the same list but each element is strict. using strictness annotation on type constructor itself should mean strictifying of all other (non-type-variable) fields: type StrictList a = !List a => data StrictList a = !Nil | !Cons !(List a) a of course, i don't mean introducing new incompatible types - that is a compiler responsibility (sorry, Simon :) ) to convert between variants of types with different strictness. That we should fix at the language definition level is what on strict types te user don't expects lazy evaluation of list/it's elements and compiler is free to use program transformations what non-lazily computes these data. for example, if putStr function accepts strict list, then it can be implemented without "evaluated?" checks on each step and the callers would ensure that all strings passed to this function are fully evaluated these two changes together should make it possible to implement strictly strict algorithms in Haskell -- Best regards, Bulat mailto:[EMAIL PROTECTED] _______________________________________________ Haskell-prime mailing list Haskell-prime@haskell.org http://haskell.org/mailman/listinfo/haskell-prime
