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MaybeBuilder not cooperating
f#compiler

I'm copying the example from page 235 of Expert F# and cannot figure out why the compiler does not like the following code. All i've done is change the type from Attempt to Maybe so it corresponds to the Haskell maybe monad.

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#light

type Maybe<'a> = (unit -> 'a option)

let succeed x = (fun () -> Some(x)) : Maybe<'a>
let fail      = (fun () -> None) : Maybe<'a>
let runMaybe (a : Maybe<'a>) = a()
let bind p rest = match runMaybe p with None -> fail | Some r -> (rest r)
let delay f = (fun () -> runMaybe (f ()))

type MaybeBuilder() =
    member b.Return(x) succeed x
    member b.Bind(p, rest) bind p rest
    member b.Delay(f) delay f
    member b.Let(p, rest) : Maybe<'a> = rest p
    
let maybe = new MaybeBuilder()

On the member declaration for b.Bind i get the following compiler error

"Unexpected incomplete structured construct at or before this point in member definition. Expected 'with', '=', or other token."

.

It looks like you're just missing equal signs:

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type MaybeBuilder() =
    member b.Return(x) = succeed x
    member b.Bind(p, rest) = bind p rest
    member b.Delay(f) = delay f
    member b.Let(p, rest) : Maybe<'a> = rest p

Also, if you have F# 1.9.6.2, the "Let" member is no longer needed; see

[link:cs.hubfs.net]

By on 10/4/2008 12:42 PM ()

Thanks! I eventually figured it out, but it was driving me crazy.

By on 10/7/2008 9:40 PM ()

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