= vs ->

[D. Tweed]

On Wed, 10 Oct 2001, Mark Carroll wrote:

> On 10 Oct 2001, Ketil Malde wrote:
> (snip)
> > function definitions.  Perhaps one could have had a syntax like
> > 
> >         z a =
> >           | a == 1 -> 1
> >           | a == 2 -> 3
> > 
> > instead, as it'd make it more consisten with the case, but I suppose
> > there's a reason for it being the way it is.  The case statement is an
> (snip)
> 
> Ah, yes - it was this 'discrepancy' that was one of the sources of my
> confusion, as "a == 1" obviously doesn't 'equal' "1".

I think this comes about from history; in the functional languages like
Miranda & Orwell that preceded Haskell an extended version of the function
above would have been written

  z a = 1   if a==1
      = 2   if a==2
      = 3   otherwise

which looks a lot like traditional mathematics and where the equals makes
sense. I'm not sure why anymore but Haskell changed the `if clause after
the value' to `pattern guard | before =', so I agree it now looks as if
it's stating that the pattern guard is equal to the rhs. 

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