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POImplementing the Haskell-MaybeMonad in F# - how can we get this lazy?
 

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  1. POImplementing the Haskell-MaybeMonad in F# - how can we get this lazy?
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    <p>we are trying to build the Haskell-MaybeMonad sample from <a href="http://www.haskell.org/all_about_monads/html/maybemonad.html" rel="noreferrer">http://www.haskell.org/all_about_monads/html/maybemonad.html</a> in F#.</p> <p>The idea is to search for a mailaddress in two dictionaries. If one of the both lookups returns a result we look into the third one. </p> <pre><code>let bindM x k = match x with | Some value -&gt; k value | None -&gt; None let returnM x = Some x type MaybeBuilder() = member this.Bind(x, k) = bindM x k member this.Return(x) = returnM x member this.ReturnFrom(x) = x member this.Delay(f) = f() let maybe = MaybeBuilder() //Sample dictionaries let fullNamesDb = [("Bill Gates", "billg@microsoft.com") ("Bill Clinton", "bill@hope.ar.us") ("Michael Jackson", "mj@wonderland.org") ("No Pref Guy", "guy@nopref.org")] |&gt; Map.ofList let nickNamesDb = [("billy", "billg@microsoft.com") ("slick willy", "bill@hope.ar.us") ("jacko", "mj@wonderland.org") ] |&gt; Map.ofList let prefsDb = [("billg@microsoft.com", "HTML") ("bill@hope.ar.us", "Plain") ("mj@wonderland.org", "HTML")] |&gt; Map.ofList let mplus m1 m2 = if m1 &lt;&gt; None then m1 else m2 let (+) = mplus let lookUp name = maybe { let! combined = fullNamesDb.TryFind name + nickNamesDb.TryFind name return! prefsDb.TryFind combined } let billGatesPref = lookUp "Bill Gates" |&gt; printfn "%A" // Some "HTML" let billyPref = lookUp "billy" |&gt; printfn "%A" // Some "HTML" let billClintonPref = lookUp "Bill Clinton" |&gt; printfn "%A" // Some "Plain" let steffenPref = lookUp "Steffen" |&gt; printfn "%A" // None let noPref = lookUp "No Pref Guy" |&gt; printfn "%A" // None System.Console.ReadKey() |&gt; ignore </code></pre> <p>The problem is that we perform the second lookup even if the first one returns a result. The nice thing about Haskell is here, that it evaluates lazy. Now we looking for something similar in F#. We tried the following but it looks ugly and seems to break the idea of encapsulating the maybe logic in the builder:</p> <pre><code>let mplus m1 m2 = if m1 &lt;&gt; None then m1 else m2() let (+) = mplus let lookUp name = maybe { let! combined = fullNamesDb.TryFind name + fun _ -&gt; nickNamesDb.TryFind name return! prefsDb.TryFind combined } </code></pre> <p>Is there a better solution?</p> <p>Regards, forki</p>
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