Re: [Haskell-cafe] Re: Wikipedia on first-class object

Thank you. data Something = This | S Something ppp :: Something -> String ppp This = "" ppp (S x) = 'S':(ppp x) How can I prevent one to pass 'let x = S x in x' to ppp ? On Sun, 30 Dec 2007 23:25:19 +0200, Chaddaï Fouché wrote:

2007/12/30, Chaddaï Fouché :

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2007/12/30, Cristian Baboi :

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A simple question:

Can you write the value of x to a file where x = (1:x) ?

Yes, but you'll have to write it yourself, because Haskell can't decide by itself that this value is infinite and try to print it with a recursive definition, if it could do that, it could decide the halting problem and that's not possible.

Sorry, I'll try to be more precise since Cristian is pretty sticky with details when he can use them in its sense...

In the particular case of x = (1:x), it can be detected that the structure is circular in memory and so infinite, but in the general case (for example if you replace (1:x) by (1:f x)) it can't be decided if the value is finite or infinite. You can't do a printer that do what you want in Haskell itself but you could probably do it with a VM or with compiler support, but it would be extremely tricky, also it would _always_ use the "running" form, even if this form isn't the optimal for storage (for example a filter of a huge list that results in an empty list would take enormously more place than with a normal printer).

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