Curried functions in JavaScript (how to employ genericly, and invisibly)?

[svend]

Hey there,

I was thinking about some curried functions in JavaScript, and I made
this:

<script>
function add(a, b, c) {
if (arguments.length == 1) {
return function (d,e) {return a + d + e}
}
if (arguments.length == 2) {
return function (d) {return a + b + d}
}
return a + b + c;
}

t = add(1,2);
alert(t(3));

alert(add(1,2,3));
</script>

Which works, but it's ugly, to say the least. So I was wondering, if
there's a way, to create this, in a more generic way? I'm aware, that
it probably wont be SML like curried, which is totally transparent,
but is there anyway at all?

In a way, so that it works, as close to SML as possible, though I'm
willing to accept, some code will be needed?

Regards,
Svend

[Douglas Crockford]

> I was thinking about some curried functions in JavaScript, and I made

this:

<script>
function add(a, b, c) {
if (arguments.length == 1) {
return function (d,e) {return a + d + e}
}
if (arguments.length == 2) {
return function (d) {return a + b + d}
}
return a + b + c;
}

t = add(1,2);
alert(t(3));

alert(add(1,2,3));
</script>

Which works, but it's ugly, to say the least. So I was wondering, if
there's a way, to create this, in a more generic way? I'm aware, that
it probably wont be SML like curried, which is totally transparent,
but is there anyway at all?

In a way, so that it works, as close to SML as possible, though I'm
willing to accept, some code will be needed?

You could write an array method sum() that would take an array and return its
sum. Then you can capture arguments as an array and apply sum to it to get a
value. That might be a little bit prettier.

http://www.crockford.com/javascript/little.html

[Dom Leonard]

Svend Tofte wrote:
<snip> I ...
was actually

thinking of something even more generic. Something, like an array, and
could return a new function, which would call the original function..

ala:

function x() {
return new function() {x(argumentsArray)}
}
Returning a function, that would call "x", with the arguments, passed
to the first call of "x", in the right order, and then apply the new
arguments, afterwards.

Function .length property and .apply method might be useful, ala

function makeCurry(func,oldArgs)
{
var length=func.length;
var oldLength = oldArgs ? oldArgs.length : 0;
function curry()
{
var i;
var myLength=arguments.length;
var newArgs;
if((oldLength==0) && (myLength >= length))
return func.apply(this,arguments);
if(myLength==0)
return curry; // no change
newArgs = new Array();
for(i=0; i < oldLength; ++i)
newArgs.push(oldArgs[i]);
for(i=0; i < myLength; ++i)
newArgs.push(arguments[i]);
return (newArgs.length >= length) ?
func.apply(this,newArgs) :
makeCurry(func,newArgs) ;
};
return length ? curry : func;
}

// as tested in Mozilla

function add(a,b,c)
{
return a+b+c;
}
var cAdd=makeCurry(add);
cAdd(2,4,8); //= 14
cAdd(2)(4)(8) //= 14
cAdd(2,4)(8) //= 14
cAdd(2)(4,8) //= 14
cAdd(2)()(4)()(8) //= 14

=======

When working up the code I ran into problems with the arguments array
not being an instance of javascript's Array constructor (I think), so
simply almalgamated old and new arguments in a loop.

HTH,
Dom

[Douglas Crockford]

> When working up the code I ran into problems with the arguments array

not being an instance of javascript's Array constructor (I think), so
simply almalgamated old and new arguments in a loop.

That is really annoying, isn't it? A coding mistake in Netscape was reverse
engineered and copied into JScript, and then written into the standard.

This will produce a real array from arguments :

Array.prototype.slice.apply(arguments)

You can also give slice additional parameters for removing some initial
parameters from the array at the same time.

http://www.crockford.com/

[Fox]

If I understand the material correctly:

function
Curry()
{
this.accumulator = 0;

function curry(arry)
{
var f = arry[0];
var count = 1;
while(count < arry.length)
{
f(arry[count++]);
}
}

if(arguments.length > 1)
{
curry(arguments);
}

return this.accumulator;
}

// test:

function
add(n)
{
this.accumulator += n;
}

function
mult(n)
{
if(this.accumulator == 0)
this.accumulator = 1;
this.accumulator *= n;
}

alert(Curry(add, 1, 2, 3, 4, 5));
alert(Curry(mult, 1, 2, 3, 4, 5));

or you can just inline the functions:

Curry(function(i){ this.accumulator += i; }, 1,2,3,4,5);
etc...
************************************************


svend wrote:

Hey there,

I was thinking about some curried functions in JavaScript, and I made
this:

<script>
function add(a, b, c) {
if (arguments.length == 1) {
return function (d,e) {return a + d + e}
}
if (arguments.length == 2) {
return function (d) {return a + b + d}
}
return a + b + c;
}

t = add(1,2);
alert(t(3));

alert(add(1,2,3));
</script>

Which works, but it's ugly, to say the least. So I was wondering, if
there's a way, to create this, in a more generic way? I'm aware, that
it probably wont be SML like curried, which is totally transparent,
but is there anyway at all?

In a way, so that it works, as close to SML as possible, though I'm
willing to accept, some code will be needed?

Regards,
Svend

[Lasse Reichstein Nielsen]

sv********@svendtofte.com (svend) writes:

In a way, so that it works, as close to SML as possible, though I'm
willing to accept, some code will be needed?

How about this currying function:
---
function curry(f,n) { // n is number of arguments for f
return function() {
var cnt = n;
var accu = [];
var self=this;
function accumulate(args) {
for(var i=0;i<args.length;i++) {
accu[accu.length]=args[i];
cnt--;
}
if (cnt<=0) {
return (f.apply(self,accu));
} else {
return function(){return accumulate(arguments);};
}
}
return accumulate(arguments);
}
}
---
And even
Function.prototype.curry = function(n){return curry(this,n);};

Then you can write

function f(a,b,c){ return a+b+c; };

and

var g= f.curry(3);
g(1,2,3) + g(1)(2)(3) + g(1,2)(3)

/L 'the wonders of untyped languages!'
--
Lasse Reichstein Nielsen - lr*@hotpop.com
Art D'HTML: <URL:http://www.infimum.dk/HTML/randomArtSplit.html>
'Faith without judgement merely degrades the spirit divine.'

[Dom Leonard]

Douglas Crockford wrote:

This will produce a real array from arguments :

Array.prototype.slice.apply(arguments)

Thanks for the thought - whilst not using it exactly, put me on a
suitable track. FWIW, the makeCurry function reduced to

function makeCurry(func,oldArgs)
{ function curry()
{ var newArgs = arguments;
if(newArgs.length ==0)
return curry; // no change
if(oldArgs)
{ newArgs=new Array();
newArgs.push.apply(newArgs,oldArgs);
newArgs.push.apply(newArgs,arguments);
}
return (newArgs.length >= func.length) ?
func.apply(this,newArgs) :
makeCurry(func,newArgs);
}
return func.length ? curry : func;
}

Cheers,
Dom