Scaling a sequence in F#

Question

I am trying to scale a sequence by the first element of the sequence, so the first element will always be one, and then subsequent elements are a ratio of the first element to the nth element of the original sequence.

Here is my code,

       open System
       open System.Collections

           let squish1 (x:Double seq) =
                let r =  (Seq.head x:Double)
                Seq.fold (fun (xi:Double) (r:Double) -> xi/r);;

And I test on this little vector:-

                squish1 [|5.0; 1.0; 1.0; 1.0; 1.0; 1.0|];;

I have typed everything because I get this error message

normaliseSequence.fsx(9,1): error FS0030: Value restriction. The value 'it' has been >inferred to have generic type val it : (Double -> '_a -> Double) when '_a :> seq
Either make the arguments to 'it' explicit or, if you do not intend for it to be generic, >add a type annotation.

But clearly I am misunderstanding because I get the error message even with everything typed. What am I missing?

Any and all advice gratefully received. Thanks

Note that it is more idiomatic in F# to use the alias float rather than System.Double. — kvb
Yes I know. This was a design choice by our development team, due to the types of data we sometimes have to handle. — Simon Hayward
But float is simply an alias for System.Double. They're the same thing. kvb is just saying that it's more idiomatic to use the float alias in your code - it changes nothing about the behavior of your program. — Joel Mueller
Interesting, will pass your comments on to the lead developer. — Simon Hayward

Daniel Daniel · Accepted Answer · 2011-10-25T15:24:07

fold expects two more parameters, the seed value and the sequence. This works:

let squish1 (x:Double seq) =
    let r =  (Seq.head x:Double)
    Seq.fold (fun (xi:Double) (r:Double) -> xi/r) 0.0 x

However, I'm guessing you probably want map instead of fold:

let squish1 (x:Double seq) =
    let r =  (Seq.head x:Double)
    Seq.map (fun (xi:Double) -> xi/r) x

Incidentally, I would probably write it this way:

let inline squish1 (x:seq<_>) =
  let r = Seq.head x
  Seq.map (fun n -> n / r) x

Now it works for all types that support division.

Scaling a sequence in F#

1 Answers