[Haskell-beginners] beginner's type error
tom.davie at gmail.com
Fri Mar 27 09:11:03 EDT 2009
On 27 Mar 2009, at 14:01, Michael Mossey wrote:
> Peter Verswyvelen wrote:
>> You can also get rid of the parentheses like this:
>> thing n = n + fromIntegral $ round $ sqrt n
> I'm having a hard time finding an explanation of the dollar signs.
> What do they do? It looks like they break up the left-ro-right
> association of function names to arguments.
You're pretty much there!
The $ function is simply "apply":
f $ a = f a
The difference is that this version of application (as opposed to the
version written as ' ') has very very low precidence, and can be used
to essentially mean "apply this to the whole expression on my right".
Of note though, using chains of ($)s as peter did is commonly
considered bad style, instead, one should use (.) to build up a
function, and then apply it, so not:
fromIntegral $ round $ sqrt n
fromIntegral . round . sqrt $ n
Why? Because the latter one has more valid expressions and is
therefor easier to refactor. For example, in the latter one I may
deside that (round . sqrt) is a useful function in itself (lets call
it integralSqrt) and refactor:
fromIntegral . integralSqrt $ n
integralSqrt = round . sqrt
With this style, this is simply a matter of copy/paste.
> As a beginner, I love how Haskell is filled with so many good ideas,
> in many areas. The basic concept of functional programming is good,
> but also Haskell has beautiful syntax that's just pleasing to look
> at, and also has many convenient features which may not quite
> qualify as "beautiful" or "elegant" but are just convenient (still a
> worthy thing).
I'm not sure, most of the convenient things I use in Haskell are also
beautiful and elegant, did you have something in mind?
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