[Haskell-cafe] How to check object's identity?

[Xie Hanjian]

* Luke Palmer <lrpalmer at gmail.com> [2009-01-03 18:46:50 -0700]:

> 2009/1/3 Xie Hanjian <jan.h.xie at gmail.com>
> 
> > Hi,
> >
> > I tried this in ghci:
> > >Prelude> 1:2:[] == 1:2:[]
> > True
> >
> > Does this mean (:) return the same object on same input,
> 
> 
> Also, in functional programming, *every* function returns the same output
> for the same input.  That's part of the definition of function.  :-)

This is true in Haskell, but may not true in Scheme (I guess also false
in Lisp).

In DrScheme:
  
  >(eq? (cons 1 2) (cons 1 2))
  #f
  >(equal? (cons 1 2) (cons 1 2))
  #t

Although equal? treats the two as the *same*, they're different lists
because if we modify one (e.g by set-car!) the other won't be affected.

So here comes another question: when we say a function always give the
same output for the same input, what the *same* means here? ídentity
or equality?

Thanks
Jan

> 
> Luke

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-- 
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