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Understanding Active Patterns
f#

I wrote this code and it works exactly as I wanted; I just do not understand why does it works!

let (|IsInt|_|) x =

match Int32.TryParse(x) with

| true, result -> Some(result)

| false, _ -> None

let (|YearMonth|_|) (args : string[]) =

match args with

| null ->

None

| [| IsInt y; IsInt m |] ->

Some (y, m)

| _ ->

None

The part I do not understand is:

...

| [| IsInt y; IsInt m |] ->

...

Which I thought should be (int option) * (int option) but - as I needed and I am happy it is this way - it is int * int.

I seems first it matches the array against being a 2-itemed array and then executes patterns against items to see if all are matched.

This seems rational, but I need to be sure; so I can apply same pattern of understanding to other situations.

And is there a restriction on "nesting" (structural) patterns?

Thanks

.

Your understanding is correct. For partial active patterns, returning 'Some' indicates that the pattern matches, whereas 'None' means it is not a match. The "option-ness" does not leak into the use site.

There's no nesting restriction.

Pattern-matching is just swell :)

By on 10/21/2010 3:08 PM ()

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