Wherein Javascript borrows a language feature from Arc

http://twoguysarguing.wordpress.com/2009/11/04/placeholders-and-currying/

by youngnh • 17 years ago

23 20 17 years ago

twoguysarguing.wordpress.com

tlrobinson3• 17 years ago

You should probably assign an object to the underscore, otherwise any argument which is undefined will match:

    js> var _
    js> _ == undefined
    true

Just doing "var _ = {}" should be sufficient.

edit: however, this conflicts with Underscore.js (http://documentcloud.github.com/underscore/), but if you put an "if (typeof _ === "undefined")" around the assignment you should be ok

youngnh• 17 years ago

I actually did a === check for just that reason, since if you define _ = {}, then passing in a real object that is {} will also return true for _ == {}

tlrobinson3• 17 years ago

Nope:

    js> a = {}
    [object Object]
    js> b = {}
    [object Object]
    js> a == b
    false

== only casts between certain primitive types, it doesn't do a deep comparison:

    js> x = null
    null
    js> y = undefined
    js> x == y
    true
    js> x === y
    false

Also undefined comparisons are still true with ===:

    js> var _
    js> _ === undefined
    true

But you're right, === should be the default choice, and only use == if you have a good reason.

ubernostrum2• 17 years ago

It also conflicts with gettext libraries. The name '_' should be considered reserved for that purpose.

tlrobinson3• 17 years ago

True, but as long as _ is defined as a non-primitive type (namely an object or function) it can be used for both gettext (or underscore.js) and this currying function. All that's required for the currying is that comparison with anything other than itself is false.

adamc• 17 years ago

Not to be a jerk, but why? What if gettext isn't important to your current application? (I don't know a lot about gettext; I went and read the blurb.)

WilliamLP• 17 years ago

> geolocator.getLatLng("Forest Park, St. Louis, MO", function(center) {

> displayMapAt(center, 14);

> });

To my eyes, that solution is just fine. I don't think I'll ever be swayed to believe that adding on layers of indirection to JavaScript code for the sake of "syntactic goodness" is rationally motivated by leading to more practical power for actually solving problems.

mikeryan• 17 years ago

agree - imagine inheriting this code and then having to figure out what the underscore is doing. ugh.

Sometimes brevity isn't the right solution.

ynniv• 17 years ago

Agreed. If you're writing something in a web browser, use sane JavaScript. Every time I have done something "fancy" with a language, there has been a large hidden cost later on, be that complicated debugging sessions, new hire training, or unintended side effects. Terrible syntax/rewriting hacks in Rails are probably what convinced me that I do not like ruby. If you truly care about programmer efficiency, you probably want to look into cross compilers like Objective-J or parenscript.

cwp• 17 years ago

See also Clamato. http://clamato.net/

cabalamat• 17 years ago

I agree. The beauty of software systems is they can be made as complex as the programmer can imagine; this is also the horror of software systems.

All software should be as complex as it needs to be to do the job, and no more so (where complex means the cognitive load necessary to understand it). Unfortunately 99% of programmers and 100% of programmers' managers don't understand this (both numbers are probably correct to the nearest integer, in my experience).

mdemare• 17 years ago

Clojure has #(+ % 10) for Arc's [+ _ 10]. Does anyone know if Arc inspired Clojure, or whether there's prior art for this syntax?

Zak• 17 years ago

Goo appears to have something similar in its op (partial evaluation) function.

jimbokun• 17 years ago

I think Clojure copied Arc on this.

richhickey• 17 years ago

Actually it was inspired by Mathematica:

http://reference.wolfram.com/mathematica/ref/Slot.html

scott_s2• 17 years ago

I think the convention predates both. boost::bind for C++ does something similar: http://www.boost.org/doc/libs/1_40_0/libs/bind/bind.html

tomjen2• 17 years ago

At least the _ part is used in both Haskell and Prolog.

scott_s2• 17 years ago

Can someone explain why this:

  geolocator.getLatLng("Forest Park, St. Louis, MO", displayMapAt);

Is better than this?

  displayMapAt(geolocator.getLatLng("Forest Park, St. Louis, MO"));

That is, why is it more sensible for getLatLng to accept a callback function that will accept its result, when one can just pass its result to a function?

abecedarius• 17 years ago

I expect the call to geolocator requires a network round-trip, with displayMapAt getting scheduled to be called when the result appears; meanwhile your code can do other things. (I'm not familiar with this API, though; I could be wrong.)

Another way to do this sort of thing, that might feel more composable: have geolocator.getLatLng() return a 'promise' object that will be asynchronously resolved.

Wherein Javascript borrows a language feature from Arc

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