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Partition a sequence until a predicate is satiated

This function is given a partition predicate and a sequence. Until the predicate returns false, a list will be filled with elements. When it is, both the list and the remainder of the sequence will be returned. Note that this example preserves the laziness of the unchecked sequence elements.

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module Seq =
    let paritionWhile (pred : _ -> bool) (sequence : _ seq) : _ list * _ seq = 
        let en = sequence.GetEnumerator ()
        let wasGood = ref true
        let sublist = 
            [
                while !wasGood && en.MoveNext() do
                    if pred en.Current then yield en.Current
                    else wasGood := false
            ]
        let remainder = 
            seq { 
                    if not !wasGood then yield en.Current
                    while en.MoveNext() do yield en.Current 
                }
        sublist, remainder

[<Fact>]
let subsetUntil_ShouldReturnProperSubset () =
    let testSeq = seq { for i in 1 .. 6 do yield i }
    let sub, remainder = testSeq |> Seq.partitionWhile (fun x -> x <= 3)
    Assert.Equal( [1; 2; 3], sub )
    Assert.Equal( [4; 5; 6], remainder |> Seq.toList )

[<Fact>] 
let subsetUntil_shouldReturnEmptyListandSeqWhenEmptyGiven () =
    let testSeq = Seq.empty
    let sub, remainder = testSeq |> Seq.partitionWhile (fun x -> x <= 3)
    Assert.Empty sub
    Assert.Empty remainder

module Seq

from Microsoft.FSharp.Collections

val paritionWhile : pred:('a -> bool) -> sequence:seq<'a> -> 'a list * seq<'a>

Full name: Script.Seq.paritionWhile

val pred : ('a -> bool)

type bool = System.Boolean

Full name: Microsoft.FSharp.Core.bool

val sequence : seq<'a>

Multiple items
val seq : sequence:seq<'T> -> seq<'T>

Full name: Microsoft.FSharp.Core.Operators.seq

--------------------
type seq<'T> = System.Collections.Generic.IEnumerable<'T>

Full name: Microsoft.FSharp.Collections.seq<_>

type 'T list = List<'T>

Full name: Microsoft.FSharp.Collections.list<_>

val en : System.Collections.Generic.IEnumerator<'a>

System.Collections.Generic.IEnumerable.GetEnumerator() : System.Collections.Generic.IEnumerator<'a>

val wasGood : bool ref

Multiple items
val ref : value:'T -> 'T ref

Full name: Microsoft.FSharp.Core.Operators.ref

--------------------
type 'T ref = Ref<'T>

Full name: Microsoft.FSharp.Core.ref<_>

val sublist : 'a list

System.Collections.IEnumerator.MoveNext() : bool

property System.Collections.Generic.IEnumerator.Current: 'a

val remainder : seq<'a>

val not : value:bool -> bool

Full name: Microsoft.FSharp.Core.Operators.not

val subsetUntil_ShouldReturnProperSubset : unit -> 'a

Full name: Script.subsetUntil_ShouldReturnProperSubset

val testSeq : seq<int>

val i : int

val sub : obj

val remainder : obj

Multiple items
module Seq

from Script

--------------------
module Seq

from Microsoft.FSharp.Collections

val toList : source:seq<'T> -> 'T list

Full name: Microsoft.FSharp.Collections.Seq.toList

val subsetUntil_shouldReturnEmptyListandSeqWhenEmptyGiven : unit -> 'a

Full name: Script.subsetUntil_shouldReturnEmptyListandSeqWhenEmptyGiven

val testSeq : seq<'b>

val empty<'T> : seq<'T>

Full name: Microsoft.FSharp.Collections.Seq.empty

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More information

Link:	http://fssnip.net/1I
Posted:	13 years ago
Author:	Rick Minerich
Tags:	seq , list , subset , partition