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[Caml-list] GC and file descriptors
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Dustin Sallings
- David Fox
- Nicolas George
- Mikael Brockman
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Date: | 2003-11-19 (02:26) |
From: | Brian Hurt <bhurt@s...> |
Subject: | Design by Contract, was Re: [Caml-list] GC and file descriptors |
On Wed, 19 Nov 2003, Martin Berger wrote: > > Well, exceptions are 'really' wrong: they're 'really' a constraint > > on the type of the argument, for example > > > > divide: float -> float not zero -> float > > > > but expressed negatively (throws divide by zero). > > that's one way of looking at it. another would be to say > we have dependent types ... unfortunatly neither rich > specifications nor type dependencies lead to decidable > type inference so we need to be less precise. > > martin > This actually brings to mind another way to improve Ocaml: Contracts, ala eiffle. The problem in the above example is that the constraint that the second argument not be zero is a contract. A classic example of a contract is Array.get, which requires the index to be >= 0 and < the length of the array. Being able to hoist this check out of Array.get can lead to non-trivial optimization opportunities. For example, consider the following code: for i = 0 to n do a.(i) <- 0 done This gets compiled like: for i = 0 to n do if i < Array.length a then a.(i) <- 0 else raise Invalid_argument "Array.get" done Strength reduction can then be applied to eliminate the redundant checks: let limit = min n ((Array.length a) - 1) in for i = 0 to limit do a.(i) <- 0 (* no check needed! *) done; if n >= (Array.length a) then raise Invalid_argument "Array.get" else () With arrays, you could simply declare them part of the language that the compiler knows about. But I'd like a more general approach. -- "Usenet is like a herd of performing elephants with diarrhea -- massive, difficult to redirect, awe-inspiring, entertaining, and a source of mind-boggling amounts of excrement when you least expect it." - Gene Spafford Brian ------------------- To unsubscribe, mail caml-list-request@inria.fr Archives: http://caml.inria.fr Bug reports: http://caml.inria.fr/bin/caml-bugs FAQ: http://caml.inria.fr/FAQ/ Beginner's list: http://groups.yahoo.com/group/ocaml_beginners