F# running on the iPhone

I tried F# on the iPhone (via MonoTouch) tonight.  The trivial hello-world code seemed to work just fine.  I used standalone to build the (trivial) fsharp code:

type Foo() =
  do System.Console.WriteLine("This output brought to you by the note F#");

mono ../FSharp-1.9.6.16/bin/fsc.exe -a Foo.fs

And added a line to instantiate a Foo in FinishedLaunching().

Loaded the resulting dll into MonoDevelop, along with FSharp.Core.dll, and it just worked.  Or at least it worked in the simulator, haven't tried the real device yet.

Sequence of positions in a sequence (F#)

Steve Horsfield's blog had a post on fold functions that cared about the beginning, middle and ends of the sequence they were operating on.  I think there’s a preprocessing step that simplifies the problem – transform the sequence into a seq<Position<'t>>, where Position tells you if the object is at the beginning, middle, or end of the sequence:

type Position<'t> =
| Beginning of 't
| Middle of 't
| End of 't
| Only of 't

let rec sequenceOfPositionsWithLazyList(existing: LazyList<'t>) =
  seq {
    let rec elementsAfterBeginning(s) =
      seq {
        match s with
        | LazyList.Cons(h, LazyList.Nil) -> yield End(h); ()
        | LazyList.Cons(h, t) -> yield Middle(h); yield! elementsAfterBeginning(t)
        | _ -> ()
      }
    match existing with
      | LazyList.Cons(h, LazyList.Nil) -> yield Only(h); ()
      | LazyList.Cons(h, t) -> yield Beginning(h); yield! elementsAfterBeginning(t)
      | LazyList.Nil -> ()
  }
  
let sequenceOfPositions(existing: seq<'t>) = sequenceOfPositionsWithLazyList(LazyList.of_seq existing)
  
printfn "%A" (sequenceOfPositions [1;2;3])
printfn "%A" (sequenceOfPositions [1;3])
printfn "%A" (sequenceOfPositions [1])
printfn "%A" (sequenceOfPositions [])

The output is:

seq [Beginning 1; Middle 2; End 3]
seq [Beginning 1; End 3]
seq [Only 1]
seq []

I think that makes following steps much easier to write.