Seq.delay<'T> Function (F#)

Article
04/28/2015

Returns a sequence that is built from the given delayed specification of a sequence.

Namespace/Module Path: Microsoft.FSharp.Collections.Seq

Assembly: FSharp.Core (in FSharp.Core.dll)

// Signature:
Seq.delay : (unit -> seq<'T>) -> seq<'T>

// Usage:
Seq.delay generator

Parameters

generator
Type: unit -> seq<'T>

The generating function for the sequence.

Return Value

The resulting sequence.

Remarks

The input function is evaluated each time an IEnumerator for the sequence is requested.

This function is named Delay in compiled assemblies. If you are accessing the function from a language other than F#, or through reflection, use this name.

Example

The following code shows how to use Seq.delay to delay the evaluation of a sequence that is created from a collection that is normally evaluated immediately.

// Normally sequences are evaluated lazily.  In this case, 
// the sequence is created from a list, which is not evaluated 
// lazily. Therefore, without Seq.delay, the elements would be 
// evaluated at the time of the call to makeSequence. 
let makeSequence function1 maxNumber = Seq.delay (fun () ->
    let rec loop n acc =
        printfn "Evaluating %d." n
        match n with
        | 0 -> acc
        | n -> (function1 n) :: loop (n - 1) acc
    loop maxNumber []
    |> Seq.ofList)
printfn "Calling makeSequence." 
let seqSquares = makeSequence (fun x -> x * x) 4          
let seqCubes = makeSequence (fun x -> x * x * x) 4
printfn "Printing sequences."
printfn "Squares:"
seqSquares |> Seq.iter (fun x -> printf "%d " x)
printfn "\nCubes:"
seqCubes |> Seq.iter (fun x -> printf "%d " x)

Output

Calling makeSequence.
Printing sequences.
Squares:
Evaluating 4.
Evaluating 3.
Evaluating 2.
Evaluating 1.
Evaluating 0.
16 9 4 1 
Cubes:
Evaluating 4.
Evaluating 3.
Evaluating 2.
Evaluating 1.
Evaluating 0.
64 27 8 1

The following code example is equivalent to the previous example, except that it does not use Seq.delay. Notice the difference in the output.

// Compare the output of this example with that of the previous. 
// Notice that Seq.delay delays the 
// execution of the loop until the sequence is used. 
let makeSequence function1 maxNumber =
    let rec loop n acc =
        printfn "Evaluating %d." n
        match n with
        | 0 -> acc
        | n -> (function1 n) :: loop (n - 1) acc
    loop maxNumber []
    |> Seq.ofList
printfn "Calling makeSequence." 
let seqSquares = makeSequence (fun x -> x * x) 4          
let seqCubes = makeSequence (fun x -> x * x * x) 4
printfn "Printing sequences."
printfn "Squares:"
seqSquares |> Seq.iter (fun x -> printf "%d " x)
printfn "\nCubes:"
seqCubes |> Seq.iter (fun x -> printf "%d " x)

Output

Calling makeSequence.
Evaluating 4.
Evaluating 3.
Evaluating 2.
Evaluating 1.
Evaluating 0.
Evaluating 4.
Evaluating 3.
Evaluating 2.
Evaluating 1.
Evaluating 0.
Printing sequences.
Squares:
16 9 4 1 
Cubes:
64 27 8 1

Platforms

Windows 8, Windows 7, Windows Server 2012, Windows Server 2008 R2

Version Information

F# Core Library Versions

Supported in: 2.0, 4.0, Portable

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