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Pattern matching - This rule will never be matched
f#compilerbook

Hi all,

I am new to F# (started this week-end), so I hope my question is not too stupid...

I am trying to generalize an exemple of Robert Pickering's book and am baffled by the following compiler warning :
This rule will never be matched.

What am I doing wrong ? (It works fine if I remove the 2nd parameter of FindHim and hardcode it as in the book)

Here is my code : 

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////////////////////
type Couple = { Him: String ; Her: String }
let couples = [ { Him = "Roméo" ; Her = "Juliette"} ; 
                      { Him = "Abélard" ; Her = "Héloïse"} ;
                      { Him = "Paul" ; Her = "Virginie" } ;
                      { Him = "Harry" ; Her = "Dors" }
                   ]

let rec FindHim l (him: String) =
    match l with
    | { Him = him ; Her = x } :: tail -> x
    | _ :: tail -> (FindHim tail him) // <b>Compiler warning here <---</b>
    | [] -> ""
/////////////////////

Thanks in advance,

Philippe

.

Your not the first person caught out by this. In fact, when you use a pattern match in this way the field in the record will get bound into the him identifier rather than testing for equality. You need add a when guard if you want a dynamic check for equality:

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open System

type Couple = { Him: String ; Her: String }
let couples = [ { Him = "Roméo" ; Her = "Juliette"} ; 
                      { Him = "Abélard" ; Her = "Héloïse"} ;
                      { Him = "Paul" ; Her = "Virginie" } ;
                      { Him = "Harry" ; Her = "Dors" }
                   ]

let rec FindHim l x =
    match l with
    | { Him = him ; Her = her } :: tail when him = x -> her
    | _ :: tail -> (FindHim tail x) // Compiler warning here <---
    | [] -> ""

HTH,
Robert

By on 11/26/2007 6:40 AM ()

Thanks a lot, Robert!
I wouldn't have found this by myself

Philippe

By on 11/26/2007 6:47 AM ()

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