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michael rice

31 Jul 2010 31 Jul '10

9:28 a.m.

From: Data.Complex data (RealFloat a) => Complex a = !a :+ !a What's the purpose of the exclamation marks? Michael

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Ivan Lazar Miljenovic

31 Jul 31 Jul

9:32 a.m.

michael rice writes:

...

From: Data.Complex

data (RealFloat a) => Complex a = !a :+ !a

What's the purpose of the exclamation marks?

Forcing; it means that the values are evaluated (up to WHNF) before the Complex value is constructed: http://www.haskell.org/ghc/docs/6.12.1/html/users_guide/bang-patterns.html -- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com IvanMiljenovic.wordpress.com

michael rice

9:55 a.m.

Thanks, Ivan. I may be back later, after I read http://en.wikibooks.org/wiki/Haskell/Laziness Michael --- On Sat, 7/31/10, Ivan Lazar Miljenovic wrote: From: Ivan Lazar Miljenovic Subject: Re: [Haskell-cafe] Constructor question To: "michael rice" Cc: haskell-cafe@haskell.org Date: Saturday, July 31, 2010, 9:32 AM michael rice writes:

...

From: Data.Complex

data (RealFloat a) => Complex a = !a :+ !a

What's the purpose of the exclamation marks?

Ben Millwood

9:55 a.m.

On Sat, Jul 31, 2010 at 2:32 PM, Ivan Lazar Miljenovic wrote:

...

Forcing; it means that the values are evaluated (up to WHNF) before the Complex value is constructed:

http://www.haskell.org/ghc/docs/6.12.1/html/users_guide/bang-patterns.html

Actually, this isn't a bang pattern: the 'a' here is not a pattern, it's a type variable. Strictness flags in data declarations are a haskell98 feature. See: http://www.haskell.org/onlinereport/haskell2010/haskellch4.html#x10-680004.2 the paragraph on Strictness Flags a little way down that page.

Ivan Lazar Miljenovic

10:01 a.m.

Ben Millwood writes:

...

On Sat, Jul 31, 2010 at 2:32 PM, Ivan Lazar Miljenovic wrote:

...
Forcing; it means that the values are evaluated (up to WHNF) before the Complex value is constructed:

http://www.haskell.org/ghc/docs/6.12.1/html/users_guide/bang-patterns.html

Actually, this isn't a bang pattern: the 'a' here is not a pattern, it's a type variable. Strictness flags in data declarations are a haskell98 feature.

See:

http://www.haskell.org/onlinereport/haskell2010/haskellch4.html#x10-680004.2

the paragraph on Strictness Flags a little way down that page.

Ugh, yeah; I should read what links I google for before putting them in emails :s -- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com IvanMiljenovic.wordpress.com

michael rice

10:22 a.m.

This Joni Mitchell lyric just popped into my head: "I've looked at clouds from both sides now

...

From up and down, and still somehow It's cloud illusions I recall I really don't know clouds at all"

A LOT of cool stuff here, but the learning curve is murder. Michael --- On Sat, 7/31/10, Ivan Lazar Miljenovic wrote: From: Ivan Lazar Miljenovic Subject: Re: [Haskell-cafe] Constructor question To: "michael rice" Cc: haskell-cafe@haskell.org Date: Saturday, July 31, 2010, 9:32 AM michael rice writes:

...

From: Data.Complex

data (RealFloat a) => Complex a = !a :+ !a

What's the purpose of the exclamation marks?

michael rice

11:54 a.m.

Ok, got ! and WHNF. Thanks, Michael --- On Sat, 7/31/10, Ivan Lazar Miljenovic wrote: From: Ivan Lazar Miljenovic Subject: Re: [Haskell-cafe] Constructor question To: "michael rice" Cc: haskell-cafe@haskell.org Date: Saturday, July 31, 2010, 9:32 AM michael rice writes:

...

From: Data.Complex

data (RealFloat a) => Complex a = !a :+ !a

What's the purpose of the exclamation marks?

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Ben Millwood
Ivan Lazar Miljenovic
michael rice