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Structural hashing changed in 1.9.6.16?
f#

The following code always served me well under 1.9.6.2:

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open System.Collections.Generic

let Cached f =
  let cache = new Dictionary<_,_>()
  let inline set a r = cache.[ a ] <- r; r
  fun a ->
    match cache.TryGetValue a with
             | true, result -> result
             | _ -> f a |> set a

let Cached2 f =
  fun a b -> Cached (fun (a,b) -> f a b) (a,b)

let sq =
  let sq x =
      printfn "%d * %d" x x
      x * x
    Cached sq

sq 3 |> printfn "%d"
sq 3 |> printfn "%d"

let plus =
  let plus x y =
    printfn "%d + %d" x y
    x + y
    Cached2 plus

plus 1 3 |> printfn "%d"
plus 1 3 |> printfn "%d"

Cached still works fine in 1.9.6.16, but Cached2 doesn't. I suppose it relies on structural hashing. Has something fundamentally changed here? It definitely broke my code! Any suggestions how to fix this?Wiebe

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The following does the trick:

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let Cached2 f =


  let c = Cached (fun (a, b) -> f a b)


  fun a b -> c (a,b)
By on 7/1/2009 2:26 PM ()Reply
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Thanks!Makes sense, strange it worked differently before.

By on 7/1/2009 11:03 PM ()Reply
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funny, but what does this caching mean?

(I needed to intend the Cached sq and Cached2 plus under the let (so one indentation left))

same thing as outputted:

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  let sqr x =

    printfn "%d * %d" x x

    x * x

  sqr 3 |> printfn "%d"

  plus x y =

     printfn "%d + %d" x y

     x + y

  plus 1 3 |> printfn "%d"

thought letting me peek into the outer let's however also:

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    let sqr =

      let sqr x =

        printfn "%d * %d" x x

        x * x

      sqr

    sqr 3 |> printfn "%d"

    let plust =

      let plust x y =

        printfn "%d + %d" x y

        x + y

      plust

    plust 1 3 |> printfn "%d"

works like the cached version. (didn't know yet that you can make a call directly to a let defined inside an outer let !?)

don't mind just trying to understand F#, I reckon the reason to Cache things aren't shown here, and you'll retrieve it someplace else afterwards.

(I moved to IE8.0 yesterday, for some reason this gives a different interface, not allowing me to keep the intended indentation in the above)

By on 7/2/2009 12:10 AM ()Reply

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