[Haskell-beginners] Random nub questions!
ajb at spamcop.net
ajb at spamcop.net
Sun Sep 14 23:05:13 EDT 2008
G'day.
Quoting Casey Rodarmor <caseyrodarmor at gmail.com>:
> Haskell is great!
Glad you think so. It proves you have good taste. :-)
> Why is this:
>
> (+ 1)
>
> a curried invocation of the the plus operator, but this:
>
> (+ 1 2)
>
> is an error?
The plus sign is an infix operator. Putting it in parentheses is
how you express it as a function, so if you want to apply the "plus"
function to two arguments, this is how you'd express it:
(+) 1 2
The syntax (+ 1), on the other hand, is slightly different. Even
though it looks like a curried function, it's actually not. It's a
special piece of syntax called an "operator section".
If it helps, the arguments are actually the wrong way around for it
to be a curried function. For a commutative operator like (+) this
may not make a difference, but for other operators, it does.
(>>= k), for example is not the same as ((>>=) k). Actually, it's a
shorthand for (\m -> m >>= k). If you wanted ((>>=) k), you'd use
(k >>=) instead.
> And in the same vein, why is this a negative number:
>
> (- 1)
>
> instead of a curried invocation of the subtraction operator?
This was a hard design decision to make. People want to express
negation with the minus sign, but it interferes with the operator
section syntax. The rule can be thought of as: If it looks like a
negation, then it is, otherwise it's an operator section.
> Why can't I do this:
>
> 1 `(+)` 2
>
> even if it is a little pathological?
The backquote notation is for turning _identifiers_ into operators,
not general expressions. It would be nice to have a syntax for
turning general expressions into operators, but Haskell doesn't have
one.
> And this might be a little implementation dependent, although I'll hazard it
> anyways: Why can't I assign to things when running GHCi?
You can do this with let syntax:
Prelude> let x = product [1..10]
Prelude> x+2
3628802
Cheers,
Andrew Bromage
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