Löb and möb: strange loops in Haskell

Re:Löb and möb: strange loops in Haskell by lalaithion on Monday January 30, 2023 @02:52PM (#34585454)

Pretty hard. The key to this implementation is Haskell's laziness, so you have to implement laziness in Javascript, which will probably not be very ergonomic. You can write

    let loeb = (structure => {
      let inner = () => structure.map(item => item(inner()));
      return inner;
    })
  
    let fs = [ _ => 1
             , x => x[0] + 1
             , x => x[1] + 1
             , x => x[2] + 1
             ]
 
    console.log(loeb(fs)())

and get an infinite loop, or

    let loeb = (structure => {
      let inner = () => structure.map(item => item(inner));
      return inner;
    })
  
    let fs = [ _ => 1
             , x => x[0] + 1
             , x => x[1] + 1
             , x => x[2] + 1
             ]
 
    console.log(loeb(fs)())

and get [1, null, null, null].

[ Parent | on HN ]

Re:Löb and möb: strange loops in Haskell by contravariant on Monday January 30, 2023 @05:32PM (#34587476)

You can typically simulate laziness by lifting variables into functions. A 0-arity function works fine, but for lists it's slightly more elegant to define them as functions from an index to a value (in that case the functor map is just composition, which is neat).

So the following works

    let loeb = (structure => {
      let inner = (i) => structure(i)(inner);
      return inner;
    })

    let fs = (i) => {switch(i) {
      case 0: return _ => 1
      case 1: return (x) => x(0) + 1
      case 2: return (x) => x(1) + 1
      case 3: return (x) => x(2) + 1
    }}

If javascript was sensible you could just do

    let fs = [ _ => 1
             , x => x(0) + 1
             , x => x(1) + 1
             , x => x(2) + 1
             ].at

but alas that doesn't work, you can do

    let fs = (i) => [ _ => 1
             , x => x(0) + 1
             , x => x(1) + 1
             , x => x(2) + 1
             ][i]

though. In that case javascript may end up rebuilding the list every time, I'm not quite sure.

[ Parent | on HN ]