JavaScript (es6) haskel where alternative

Question

In Haskell you can create where condition for temporary variables like this:

f x
  | cond1 x   = a
  | cond2 x   = g a
  | otherwise = f (h x a)
  where
    a = w x

Is it possible to create this in javascript but with expression not statements.

For example:

let a = 10;
let b = a + 20;

return a + b

This is just simple example which doesn't require temporary variables but it was just example.

The below example is with statements - but I wonder if there is good alternative with expression.

Ramdajs can be used if it appropriate.

Thanks

`where` is always local to a function, and sure you can easily put a declaration inside a function — Bergi, Dec 17 '19 at 10:45
"*The below example is with statements*" - what example are you referring to? — Bergi, Dec 17 '19 at 10:45
Notice that it's a statement in Haskell as well, not an expression (`let a = w x in …`) — Bergi, Dec 17 '19 at 10:46
If i have R.[function](R.{function}, R.{function})((a,b) => {code}) - in the place of code I want to put expression because it's less code and return directly instead of creating {let a =10,let b=20;code; return result } — , Dec 17 '19 at 10:49
I still fail to see how using statements would be undesirable. — Aadit M Shah, Dec 17 '19 at 10:53

score 1 · Answer 1 · answered Dec 17 '19 at 10:37

1

IIFEs would work:

  (a => (b => a + b)(a + 20))(10)

answered Dec 17 '19 at 10:37

Jonas Wilms

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Or more legibly, `((a = 10, b = a + 20) => a + b)()`. – Aadit M Shah Dec 17 '19 at 10:43
Thanks, it works but i find it hard to read if i have 3, 4 variables, i didn't find anything better anyway – Dec 17 '19 at 10:46
@AaditMShah Don't abuse default initialisers for that – Bergi Dec 17 '19 at 10:46
@Bergi How about `(() => { const a = 10; const b = a + 20; return a + b; })()` then? – Aadit M Shah Dec 17 '19 at 10:49
It's very long as code, I want something easy like in haskell but maybe it doesn't exists in js – Dec 17 '19 at 10:52
@user2693928 A long piece of code does not imply that it's not easy. – Aadit M Shah Dec 17 '19 at 10:55
@aadit that ain't expressions only. – Jonas Wilms Dec 17 '19 at 10:56
1

@JonasWilms Sure, but the entire thing is an expression. I don't understand the fascination with everything being an expression anyway. In fact, as Bergi pointed out the `where` clause in Haskell is also not an expression. – Aadit M Shah Dec 17 '19 at 10:58
IMHO, readability trumps "everything should be an expression". – Aadit M Shah Dec 17 '19 at 10:59
"What you ask for is what you get" ... :) – Jonas Wilms Dec 17 '19 at 11:02
1

@AaditMShah I like that IIFE with `const` declarations much better, thanks :-) – Bergi Dec 17 '19 at 11:39
1

@AaditMShah Also remember [How to simulate let expressions in JavaScript?](https://stackoverflow.com/q/51407631/1048572) :-) – Bergi Dec 17 '19 at 11:42
@Bergi Ahaha. Yes, I do. I was really naive last year. =) – Aadit M Shah Dec 17 '19 at 12:36

score 0 · Answer 2 · answered Dec 17 '19 at 10:50

You can use Javascript's default parameters in a creative way:

const _let = f => f();

const main = _let((a = 10, b = a + 20) =>
  a + b);

console.log(main);

Please note that default parameters aren't evaluated recursively, i.e. the left parameter cannot depend on the right one.

tevemadar · Answer 3 · 2019-12-17T10:56:28.790

0

The closest thing is a lambda / arrow function, invoked on spot.

Example from Haskell: Where vs. Let (because I do not know Haskell)

f x y  |  y>z           =  ...
       |  y==z          =  ...
       |  y<z           =  ...
     where z = x*x

Could be something like

((x,y)=>{
  let z=x*x;
  if(y>z){console.log("y>z");}
  else if(y==z){console.log("y==z");}
  else /*if(y<z)*/{console.log("y<z");}
})(10,20);

Well, if I understand correctly that f x y | ... is some kind of a switch statement with predicates (which do not really convert to the switch of JS)

edited Dec 17 '19 at 10:56

answered Dec 17 '19 at 10:53

tevemadar

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Thanks, I will write it with statements, that way is many characters and hard to read – Dec 17 '19 at 10:55

JavaScript (es6) haskel where alternative

3 Answers3