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Trying to understand a snippet of F# found in the Active Patterns Paper
f#

In the 'active patterns' paper by Don Syme, et al.

[link:blogs.msdn.com]

There is this snippet of F# relating to Xml processing using active patterns on page 8:

=======================

let rec (|ShapeElem|_|) inp =

match inp with

| Elem "Sphere"

(Attributes (Attr "r" (Float r) &

Vector (x,y,z)))

-> Some (Sphere (r,x,y,z))

| Elem "Intersect" (ShapeElems(objs))

-> Some (Intersect objs)

| _ -> None

and (|ShapeElems|) inp =

[ for (ShapeElem y) in inp.ChildNodes -> y ]

========================

I am confuse about the 'and' towards the end of the snippet - it seems to be at the same level as 'let'.

I am assuming the code is correct as it seems to compile in F# interactive but I can't make sense of the 'and'.

Can someone please explain?

Thanks,

Faisal

.

"let rec ... and ..." defines mutually recursive bindings.

[link:msdn.microsoft.com]

By on 4/4/2010 9:05 PM ()

Thanks Brian.

By on 4/5/2010 12:51 PM ()

"and" is just shorthand for another lexically scoped "let" in this case.

If you were using Ocaml syntax, both would be

let ... in

let ... in

i.e., in this case,

let ...

and ... (in)

Anyone: feel free to correct me.

But I think your intuition about the matter (that "and" is lexically

equivalent to "let") is not only observationally correct, but also

syntactically and semantically correct.

I say this because the function signatures are *different* (rec (|ShapeElem|_) vs. (|ShapeElems|_|)...). Were they

the same, I'd be confused as well. :)

By on 4/4/2010 7:42 PM ()

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