powerset

[Liyang HU]

On Thu, Jun 05, 2003 at 10:03:55AM +0100, Graham Klyne wrote:
> At 19:50 04/06/03 -0400, Derek Elkins wrote:
> > You (Graham) also have some parentheses issues; e.g. in foo ++
> > (combinations 5 l) the parentheses are superfluous.
> I'm tempted to argue that being superfluous doesn't mean they
> shouldn't be there.

Keith already made one argument for less parentheses, here's my take on
the subject:

Given that:
    (++)   :: [a] -> [a] -> [a]  -- contatenates lists
    foo, l :: [a]                -- are lists
    bar    :: Int -> [a] -> [a]  -- produces a list

Which of the following make sense?
> (foo ++) bar 5 l  -- No, because bar isn't a list,
> (foo ++ bar) 5 l  --  and 5 and l would be applied to a list,
> (foo ++ bar 5) l  --  (as opposed to a function,) which make no sense

> foo (++ bar 5 l)  -- Can't apply a function to a value,
>   --  though (++ bar 5 l) foo has the same effect as what we intended

> foo ++ (bar 5 l)  -- which is just foo ++ bar 5 l

Because of the static type checking that takes place, you can't easily
(not unless you were _trying_ ;) produce an ambiguous expression such
that the removal of brackets keeps it well-typed, yet is not equivalent
to the original. So as opposed to the C code where you (and I) would put
in extra brackets `just to be sure', I wouldn't bother with them unless
I know the expression's going to be ambiguous. (or if the compiler tells
me so. ;-)

(I suppose you could argue it's not necessarily obvious that foo is a
list and bar is a 2-ary function of an Int and a list. My response would
be to rename foo and bar so that this is the case. ;-)

> I'd just about figured the ShowS idea, but I've yet to get a handle on this 
> idea of [a] 'monoid'.

Might http://www.engr.mun.ca/~theo/Misc/haskell_and_monads.htm be of any
help?

later,
/Liyang -- who managed to sneak into Category Theory lectures, but still
has no idea what a monad is. ^_^;
-- 
.--| Liyang HU |--| http://nerv.cx/ |--| Caius@Cam |--| ICQ: 39391385 |--.
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