[Haskell-cafe] Why is $ right associative instead of leftassociative?

Chris Kuklewicz haskell at list.mightyreason.com
Sat Feb 4 18:34:41 EST 2006

Brian Hulley wrote:
> Jared Updike wrote:
>>>         [a,b,c ; tail]      ===      a :: b :: c :: tail         --
>>> where ::
>> How is [a,b,c ; tail]   simpler, clearer or less typing than
>> a:b:c:tail  ? I think that the commas and semicolons are easy to
>> confuse.
> It seems strange that you can write [a,b,c] with a nice list sugar but
> if you want to include a tail you have to switch to the infix notation
> using list cons. Prolog for example allows you to write [a,b,c|Tail] but
> neither Haskell nor ML allows this. In Haskell, | is used to introduce a
> list comprehension so I was just trying to find a replacement symbol for
> when you want the equivalent of the Prolog list sugar so that you
> wouldn't be forced to use infix notation.
> All this was not to replace a:b:c:tail but was to replace a::b::c::tail
> so that : could be used for type annotations instead.

There is the .. operator which is unused in pattern matching contexts. So maybe

case [1,3..] of
  [a,b,c,tail..] -> tail   -- I like this one, the ..] catches the eye better
  [a,b,c,..tail] -> tail   -- I think this is less clear at a glance
  [a,b,c,..tail..] -> tail -- I expect no one to like this
  [a,b,c,_..] -> [a,b,c]   -- Not the best looking thing I've ever seen
  [a,b,c,.._] -> [a,b,c]   -- ditto
  [a,b,c,.._..] -> [a,b,c] -- ick

But this implies [a,b,c,[]..] is the same as [a,b,c] and [a,b,c,[d,e,f]..] is
the same as [a,b,c,d,e,f] and [a,b,c,[d,e,f..]..] is [a,b,c,d,e,f..]


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