Fold.step2 not clearly defined

[Forthoney]

The FunctionalRecordUpdate structure provided in the guides refers to the function Fold.step2. However, it is not a function in the Fold structure. The Fold guide provides step0 and step1, but it is not immediately clear from their description what step2 should be.

It is sometimes syntactically convenient to omit the parentheses around the steps in a fold. This is easily done by defining a new function, step1...

The guide seems to present step1 as a syntactic convenience. This angle makes it difficult to figure out what step2 should do based off of its predecessors.

[YawarRaza7349]

The FunctionalRecordUpdate page actually seems to be mixing up two different invocation syntaxes. The examples on the page are:

updateAB {a = 13, b = "hello"} (set#b "goodbye") $
updateAB {a = 13.5, b = true} (set#b false) (set#a 12.5) $
updateBCD {b = 1, c = 2, d = 3} (set#c 4) (set#c 5) $

These are simply step0s and the upd/set functions should instead be:

fun upd s f z = Fold.step0 (fn (vars, ops) => (fn out => vars (s (ops ()) (out, f)), ops)) z
fun set s v z = Fold.step0 (fn (vars, ops) => (fn out => vars (s (ops ()) (out, fn _ => v)), ops)) z

As you mentioned, step1/step2/etc is for obviating parentheses, for writing:

updateAB {a = 13, b = "hello"} set#b "goodbye" $
updateAB {a = 13.5, b = true} set#b false set#a 12.5 $
updateBCD {b = 1, c = 2, d = 3} set#c 4 set#c 5 $

step2 would be implemented as:

fun step2 (h: 'a11 * 'a12 * 'a13 -> 'a2)
          (a13: 'a13, f: 'b -> 'c)
          (a11: 'a11)
          (a12: 'a12): ('a2, 'b, 'c, 'd) t =
   fold (h (a11, a12, a13), f)

with the signature including:

type ('a11, 'a12, 'a13, 'a2, 'b, 'c, 'd) step2 =
   ('a13, 'b, 'c, 'a11 -> 'a12 -> ('a2, 'b, 'c, 'd) t) step

val step2: ('a11 * 'a12 * 'a13 -> 'a2)
           -> ('a11, 'a12, 'a13, 'a2, 'b, 'c, 'd) step2

You can decide whether to use the step0 version or the step2 version, but the step0 version seems clearer.

[MatthewFluet]

I didn't see anything in the commit history of either of those pages, but they haven't been edited much since the guide was converted from a regular wiki to AsciiDoc.

It sounds like updates to the FunctionalRecordUpdate page are warranted.

[YawarRaza7349]

The bottom of the current page mentions a change circa September 14, 2009. This is the version of the page before that, which uses a step0 similar to what my previous comment mentioned. The change to the next version seems connected to this mailing-list thread. And here is someone else who had found the same step2 omission as OP.