[Haskell-cafe] Re: Why purely in haskell?

[Cristian Baboi]

On Fri, 11 Jan 2008 09:16:12 +0200, Lennart Augustsson  
<lennart at augustsson.net> wrote:

> Thank you Duncan, you took the words out of my mouth. :)

> On Jan 10, 2008 5:42 PM, Duncan Coutts <duncan.coutts at worc.ox.ac.uk>  
> wrote:

>> >
>> So let's imagine:
>>
>> ones = 1 : ones
>>
>> ones' = repeat 1
>>  where repeat n = n : repeat n
>>
>> So you're suggesting that:
>>
>> ones == ones = True
>> but
>> ones' == ones' = _|_
>>
>>
>> Well if that were the case then  it is distinguishing two equal values
>> and hence breaking referential transparency. We can fairly trivially
>> prove that ones and ones' are equal so == is not allowed to distinguish
>> them. Fortunately it is impossible to write == above, at least using
>> primitives within the language.

If one can prove ones == ones = True with some method, why that method  
cannot be made to work on ones' ?

Are you thinking about repeat (f x) by any chance ?



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