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Matching type with variable binding when doing pattern matching.
f#

Sorry for my poor english

When doing pattern matching, i can match against types:

let stringType (t : obj) =
match t with
| :? string -> "String"
| :? (string * string) -> "string * string"
| _ -> "Unknown"

sometimes I want to match the type with binding it to some variables, like:

let matchWithBinding(o : obj) =
match o with
| :? string as s when s.Length < 5 -> "string with length < 5"
| _ -> "..."

but I found I can't match tuple in that way. The code below cannot be compiled:

let matchTupleWithBinding(o : obj) =
match o with
| :? (string * string) as (url, content) when url.Length < 10 ->
...

How can I do something like that?

.

Interesting, it does not seem possible ('as' only accepts an identifier, not a pattern). Here is best I could do:

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let o = box (1,"foo")

match o with

| :? (int * string) as t when (match t with x,y -> x > 3) -> match t with x,y -> printfn "%d,%s" x y

| _ -> printfn "either not the right tuple type, or was but x <= 3"

but it matches 't' twice.

By on 5/14/2009 1:23 AM ()

Hi,

It feels like there should be a better way to do this but, this would be my first thought:

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let matchTupleWithBinding(o : obj) = 
    match o with 
    | :? (string*string) when (let url,_ = (o :?> (string*string)) in url.Length < 10) ->         
        let _,content = o :?> (string*string)
        content
    | _ -> "Unknown"

As I said, I suspect there may be a way to avoid performing the cast twice. I'll come back if anything springs to mind.

Regards,

Michael

By on 5/14/2009 12:32 AM ()

Or even:

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let matchTupleWithBinding2(o : obj) = 
    match o with 
    | :? (string*string) as s when (fst s).Length < 10        ->
        snd s
    | _ -> "Unknown"

Regards,

Michael

By on 5/14/2009 12:38 AM ()

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