[Haskell-cafe] \_ - not equivalent to const $
In attempting to devise a variant of cycle which did not keep its argument alive (for the purpose of cycle [1::Int..]), I came across this peculiar behavior: import Debug.Trace cycle' :: (a - [b]) - [b] cycle' xs = xs undefined ++ cycle' xs take 20 $ cycle' (const $ 1:2:3:4:trace x 5:[]) [1,2,3,4,x 5,1,2,3,4,5,1,2,3,4,5,1,2,3,4,5] Nuts. Oh, but wait: take 20 $ cycle' (\_ - 1:2:3:4:trace x 5:[]) [1,2,3,4,x 5,1,2,3,4,x 5,1,2,3,4,x 5,1,2,3,4,x 5] Hey, it worked! Can someone explain what the heck is going on here? Luke ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] \_ - not equivalent to const $
On Jan 10, 2008 8:54 PM, Luke Palmer [EMAIL PROTECTED] wrote: Can someone explain what the heck is going on here? AFAICT, nothing is wrong. You see, both returned the very same values. What you saw was in fact the problem with unsafePerformIO and friends, as they may be executed lots of times *or not*. The compiler is free to behave in those two ways for the code with const or with the lambda. But referential transparency wasn't broken at all =). It seems const retained the value, while the lambda didn't. An optimization might transform one into the other if the compiler sees fit. -- Felipe. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] \_ - not equivalent to const $
On Jan 10, 2008 11:11 PM, Felipe Lessa [EMAIL PROTECTED] wrote: On Jan 10, 2008 8:54 PM, Luke Palmer [EMAIL PROTECTED] wrote: Can someone explain what the heck is going on here? AFAICT, nothing is wrong. You see, both returned the very same values. What you saw was in fact the problem with unsafePerformIO and friends, as they may be executed lots of times *or not*. The compiler is free to behave in those two ways for the code with const or with the lambda. But referential transparency wasn't broken at all =). Of course. I'm trying to write a variant of cycle that will be efficient with memory for my purposes, so the Haskell 98 standard, which says nothing about memory usage, doesn't really interest me. I'm more interested in what is going on in ghc (6.8.1) in this case, if it has a simple explanation. And of course am interested if there are any better ways to write such a function. Luke ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] \_ - not equivalent to const $
On Jan 11, 2008 2:11 AM, Felipe Lessa [EMAIL PROTECTED] wrote: On Jan 10, 2008 8:54 PM, Luke Palmer [EMAIL PROTECTED] wrote: Can someone explain what the heck is going on here? AFAICT, nothing is wrong. You see, both returned the very same values. [snip] But referential transparency wasn't broken at all =). Referential transparency wasn't broken, but I wonder what was the compiler, and what were it's options. -- vir http://vir.comtv.ru/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] \_ - not equivalent to const $
On Jan 10, 2008 11:15 PM, Victor Nazarov [EMAIL PROTECTED] wrote: On Jan 11, 2008 2:11 AM, Felipe Lessa [EMAIL PROTECTED] wrote: On Jan 10, 2008 8:54 PM, Luke Palmer [EMAIL PROTECTED] wrote: Can someone explain what the heck is going on here? AFAICT, nothing is wrong. You see, both returned the very same values. [snip] But referential transparency wasn't broken at all =). Referential transparency wasn't broken, but I wonder what was the compiler, and what were it's options. Ahh, it was ghc 6.8.1, without any optimization. If I turn on optimization, the behavior goes away, and they both behave like the const version. Darn. Luke -- vir http://vir.comtv.ru/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] \_ - not equivalent to const $
On Jan 10, 2008 9:20 PM, Luke Palmer [EMAIL PROTECTED] wrote: Ahh, it was ghc 6.8.1, without any optimization. If I turn on optimization, the behavior goes away, and they both behave like the const version. That was what I suggested on the last line of my previous e-mail. As const is in Prelude, which probably was compiled with optimizations, it would explain the difference. -- Felipe. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] \_ - not equivalent to const $
On Jan 11, 2008 2:28 AM, Felipe Lessa [EMAIL PROTECTED] wrote: On Jan 10, 2008 9:20 PM, Luke Palmer [EMAIL PROTECTED] wrote: Ahh, it was ghc 6.8.1, without any optimization. If I turn on optimization, the behavior goes away, and they both behave like the const version. That was what I suggested on the last line of my previous e-mail. As const is in Prelude, which probably was compiled with optimizations, it would explain the difference. Optimization of const doesn't affect this program behavior. -- vir http://vir.comtv.ru/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] \_ - not equivalent to const $
On Jan 10, 2008 11:54 PM, Luke Palmer [EMAIL PROTECTED] wrote: Can someone explain what the heck is going on here? Evaluating (const EXPR) creates a closure object with a pointer to 'const' and a pointer to the EXPR thunk. Call this closure object C. Evaluating (C undefined) calls 'const' with the EXPR and C thunks as its arguments, which returns the EXPR thunk. The EXPR thunk is then forced by your code. Evaluating (C undefined) again calls 'const' with the EXPR and C pointers as its arguments again, which again returns the *now forced* EXPR thunk. I.e., evaluating (const EXPR) creates a closure with a pointer to a single EXPR thunk, and then applying the same closure to some argument multiple times uses that same EXPR thunk every time. An unoptimized (\_ - EXPR) creates a new thunk each time the function is applied to an argument. Hope that helps with understanding what is going on? - Benja ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
