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Albert Y. C. Lai

12 Mar 2007 12 Mar '07

4:36 p.m.

main = print (map (const 'x') (take 1 (undefined:undefined))) In ghci, or with ghc -O0, this produces "x". With ghc -O, this produces Prelude.undefined. But hey, please don't sweat too much on it! I bet it is a rather pathological case.

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Kirsten Chevalier

12 Mar 12 Mar

5:08 p.m.

On 3/12/07, Albert Y. C. Lai wrote:

main = print (map (const 'x') (take 1 (undefined:undefined)))

In ghci, or with ghc -O0, this produces "x". With ghc -O, this produces Prelude.undefined.

What version of ghc? Cheers, Kirsten -- Kirsten Chevalier* chevalier@alum.wellesley.edu *Often in error, never in doubt "make them believe, if not in magic, in money well spent" -- Annie Gallup

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Kirsten Chevalier

5:53 p.m.

On 3/12/07, Kirsten Chevalier wrote:

On 3/12/07, Albert Y. C. Lai wrote:

...
main = print (map (const 'x') (take 1 (undefined:undefined)))

In ghci, or with ghc -O0, this produces "x". With ghc -O, this produces Prelude.undefined.

What version of ghc?

I was curious, so I checked this against ghc 6.6. Indeed, it exhibits the behavior Albert describes above. Same goes for the HEAD. In ghc 6.4.2, however, the program prints "x" whether compiled with -O or -O0. This does seem like a bug to me. Cheers, Kirsten -- Kirsten Chevalier* chevalier@alum.wellesley.edu *Often in error, never in doubt "Live fast, love hard, and wear corrective lenses if you need them." --Webb Wilder

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Kirsten Chevalier

13 Mar 13 Mar

5:20 a.m.

On 3/12/07, Kirsten Chevalier wrote:

On 3/12/07, Kirsten Chevalier wrote:

...
On 3/12/07, Albert Y. C. Lai wrote:

...
main = print (map (const 'x') (take 1 (undefined:undefined)))

In ghci, or with ghc -O0, this produces "x". With ghc -O, this produces Prelude.undefined.

What version of ghc?

I was curious, so I checked this against ghc 6.6. Indeed, it exhibits the behavior Albert describes above. Same goes for the HEAD. In ghc 6.4.2, however, the program prints "x" whether compiled with -O or -O0.

This does seem like a bug to me.

I noticed that compiling with -O -frules-off causes the test program here to correctly print out "x". So, I was looking at the "take" rule in GHC/List.lhs. Doesn't this rule change the strictness of take? "take" [~1] forall n xs . take n xs = case n of I# n# -> build (\c nil -> foldr (takeFB c nil) (takeConst nil) xs n#) Cheers, Kirsten -- Kirsten Chevalier* chevalier@alum.wellesley.edu *Often in error, never in doubt "It's a woman's dream, this autonomy / Where the lines connect and the points stay free" -- Ferron

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Kirsten Chevalier

6:02 a.m.

On 3/13/07, Kirsten Chevalier wrote:

I noticed that compiling with -O -frules-off causes the test program here to correctly print out "x". So, I was looking at the "take" rule in GHC/List.lhs. Doesn't this rule change the strictness of take?

"take" [~1] forall n xs . take n xs = case n of I# n# -> build (\c nil -> foldr (takeFB c nil) (takeConst nil) xs n#)

I may be talking to myself here, but what I mean is:

take 1 (1:undefined) [1] build (\ c nil -> foldr (takeFB c nil) (takeConst nil) (1:undefined) 1#) [1*** Exception: Prelude.undefined

(where takeFB and takeConst are defined as they are in GHC/List.lhs) So that rule doesn't seem to be quite right. Cheers, Kirsten -- Kirsten Chevalier* chevalier@alum.wellesley.edu *Often in error, never in doubt "80% of success is showing up."--Woody Allen

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Simon Peyton-Jones

6:31 a.m.

You are quite right. I've submitted a Trac report. Simon | -----Original Message----- | From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell-users-bounces@haskell.org] On | Behalf Of Kirsten Chevalier | Sent: 13 March 2007 10:03 | To: Albert Y. C. Lai | Cc: glasgow-haskell-users@haskell.org | Subject: Re: slight difference in strictness between -O0 and -O | | On 3/13/07, Kirsten Chevalier wrote: | > | > I noticed that compiling with -O -frules-off causes the test program | > here to correctly print out "x". So, I was looking at the "take" rule | > in GHC/List.lhs. Doesn't this rule change the strictness of take? | > | > "take" [~1] forall n xs . take n xs = case n of I# n# -> build (\c | > nil -> foldr (takeFB c nil) (takeConst nil) xs n#) | > | | I may be talking to myself here, but what I mean is: | | > take 1 (1:undefined) | [1] | > build (\ c nil -> foldr (takeFB c nil) (takeConst nil) (1:undefined) 1#) | [1*** Exception: Prelude.undefined | | (where takeFB and takeConst are defined as they are in GHC/List.lhs) | | So that rule doesn't seem to be quite right. | | Cheers, | Kirsten | | -- | Kirsten Chevalier* chevalier@alum.wellesley.edu *Often in error, never in doubt | "80% of success is showing up."--Woody Allen | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users@haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

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Albert Y. C. Lai
Kirsten Chevalier
Simon Peyton-Jones