+ zipShortest function and make generic zip safe (#370)

Author: Rafal

src/FSharpPlus/Control/Functor.fs

@@ -170,20 +170,21 @@ type Unzip =
 type Zip =
     inherit Default1
 
-    static member Zip ((x: IEnumerator<'T>            , y: IEnumerator<'U>           , _output: IEnumerator<'T*'U>           ), _mthd: Zip) = Enumerator.zip x y
-    static member Zip ((x: seq<'T>                    , y: seq<'U>                   , _output: seq<'T*'U>                   ), _mthd: Zip) = Seq.zip        x y
-    static member Zip ((x: NonEmptySeq<'T>            , y: NonEmptySeq<'U>           , _output: NonEmptySeq<'T*'U>           ), _mthd: Zip) = NonEmptySeq.zip        x y
-    static member Zip ((x: IDictionary<'K, 'T>        , y: IDictionary<'K,'U>        , _output: IDictionary<'K,'T*'U>        ), _mthd: Zip) = Dict.zip       x y
+    static member Zip ((x: IEnumerator<'T>            , y: IEnumerator<'U>           , _output: IEnumerator<'T*'U>           ), _mthd: Zip) = Enumerator.zip          x y
+    static member Zip ((x: seq<'T>                    , y: seq<'U>                   , _output: seq<'T*'U>                   ), _mthd: Zip) = Seq.zip                 x y
+    static member Zip ((x: NonEmptySeq<'T>            , y: NonEmptySeq<'U>           , _output: NonEmptySeq<'T*'U>           ), _mthd: Zip) = NonEmptySeq.zip         x y
+    static member Zip ((x: IDictionary<'K, 'T>        , y: IDictionary<'K,'U>        , _output: IDictionary<'K,'T*'U>        ), _mthd: Zip) = Dict.zip                x y
     static member Zip ((x: IReadOnlyDictionary<'K, 'T>, y: IReadOnlyDictionary<'K,'U>, _output: IReadOnlyDictionary<'K,'T*'U>), _mthd: Zip) = IReadOnlyDictionary.zip x y
     static member Zip ((x: Dictionary<'K, 'T>         , y: Dictionary<'K,'U>         , _output: Dictionary<'K,'T*'U>         ), _mthd: Zip) = Dict.zip       x y :?> Dictionary<'K,'T*'U>
     static member Zip ((x: Map<'K, 'T>                , y: Map<'K,'U>                , _output: Map<'K,'T*'U>                ), _mthd: Zip) = Map.zip        x y
     static member Zip ((f: 'R -> 'T                   , g: 'R -> 'U                  , _output: 'R -> 'T * 'U                ), _mthd: Zip) = fun x -> (f x, g x)
     static member Zip ((f: Func<'R, 'T>               , g: Func<'R, 'U>              , _output: Func<'R, 'T * 'U>            ), _mthd: Zip) = Func<_,_> (fun x -> (f.Invoke x, g.Invoke x))
-    static member Zip ((x: list<'T>                   , y: list<'U>                  , _output: list<'T*'U>                  ), _mthd: Zip) = List.zip       x y
-    static member Zip ((x: 'T []                      , y: 'U []                     , _output: ('T*'U) []                   ), _mthd: Zip) = Array.zip      x y
-    static member Zip ((x: option<'T>                 , y: option<'U>                , _output: option<'T*'U>                ), _mthd: Zip) = Option.zip     x y
-    static member Zip ((x: Async<'T>                  , y: Async<'U>                 , _output: Async<'T*'U>                 ), _mthd: Zip) = Async.zip      x y
-    static member Zip ((x: Task<'T>                   , y: Task<'U>                  , _output: Task<'T*'U>                  ), _mthd: Zip) = Task.zip       x y
+    static member Zip ((x: list<'T>                   , y: list<'U>                  , _output: list<'T*'U>                  ), _mthd: Zip) = List.zipShortest        x y
+    static member Zip ((x: 'T []                      , y: 'U []                     , _output: ('T*'U) []                   ), _mthd: Zip) = Array.zipShortest       x y
+    static member Zip ((x: ResizeArray<'T>            , y: ResizeArray<'U>           , _output: ResizeArray<'T*'U>           ), _mthd: Zip) = ResizeArray.zipShortest x y
+    static member Zip ((x: option<'T>                 , y: option<'U>                , _output: option<'T*'U>                ), _mthd: Zip) = Option.zip              x y
+    static member Zip ((x: Async<'T>                  , y: Async<'U>                 , _output: Async<'T*'U>                 ), _mthd: Zip) = Async.zip               x y
+    static member Zip ((x: Task<'T>                   , y: Task<'U>                  , _output: Task<'T*'U>                  ), _mthd: Zip) = Task.zip                x y
 
     static member inline Invoke (source1: '``ZipFunctor<'T1>``) (source2: '``ZipFunctor<'T2>``) =
         let inline call_4 (a: ^a, b: ^b, c: ^c, d: ^d) = ((^a or ^b or ^c or ^d) : (static member Zip : (_*_*_)*_ -> _) (b, c, d), a)

src/FSharpPlus/Data/NonEmptyList.fs

@@ -98,6 +98,17 @@ module NonEmptyList =
     /// <param name="list2">The second input list.</param>
     /// <returns>A single list containing pairs of matching elements from the input lists.</returns>
     let zip (list1: NonEmptyList<'T>) (list2: NonEmptyList<'U>) = {Head = (list1.Head, list2.Head); Tail = List.zip list1.Tail list2.Tail}
+    
+    /// <summary>
+    /// Zip safely two lists. If one list is shorter, excess elements are discarded from the right end of the longer list. 
+    /// </summary>
+    /// <param name="a1">First input list.</param>
+    /// <param name="a2">Second input list.</param>
+    /// <returns>List with corresponding pairs of input lists.</returns>
+    let zipShortest (list1: NonEmptyList<'T>) (list2: NonEmptyList<'U>) =
+        { Head = (list1.Head, list2.Head); Tail = List.zipShortest list1.Tail list2.Tail }
+    
+    
     /// Returns a new NonEmptyList with the element added to the beginning.
     let cons e {Head = x; Tail = xs} = {Head = e ; Tail = x::xs}
     /// Returns the first element of a new non empty list. You can also use property nel.Head.
@@ -200,8 +211,8 @@ type NonEmptyList<'t> with
     static member Unzip s = NonEmptyList.unzip s
 
     [<EditorBrowsable(EditorBrowsableState.Never)>]
-    static member Zip (x, y) = NonEmptyList.zip x y
-
+    static member Zip (x, y) = NonEmptyList.zipShortest x y
+    
     static member (>>=) ({Head = x; Tail = xs}, f: _->NonEmptyList<'b>) =
         let {Head = y; Tail = ys} = f x
         let ys' = List.collect (NonEmptyList.toList << f) xs

src/FSharpPlus/Extensions/Array.fs

@@ -86,3 +86,12 @@ module Array =
         let (x, y) = ResizeArray (), ResizeArray ()
         Array.iter (mapper >> function Choice1Of2 e -> x.Add e | Choice2Of2 e -> y.Add e) source
         x.ToArray (), y.ToArray ()
+    
+    /// <summary>
+    /// Zip safely two arrays. If one array is shorter, excess elements are discarded from the right end of the longer array. 
+    /// </summary>
+    /// <param name="a1">First input array.</param>
+    /// <param name="a2">Second input array.</param>
+    /// <returns>Array with corresponding pairs of input arrays.</returns>
+    let zipShortest (a1: array<'T1>) (a2: array<'T2>) =
+        Array.init (min a1.Length a2.Length) (fun i -> a1.[i], a2.[i])

src/FSharpPlus/Extensions/List.fs

@@ -125,3 +125,17 @@ module List =
                 | Choice1Of2 x -> loop (x::acc1, acc2) xs
                 | Choice2Of2 x -> loop (acc1, x::acc2) xs
         loop ([], []) (List.rev source)
+    
+    /// <summary>
+    /// Zip safely two lists. If one list is shorter, excess elements are discarded from the right end of the longer list. 
+    /// </summary>
+    /// <param name="a1">First input list.</param>
+    /// <param name="a2">Second input list.</param>
+    /// <returns>List with corresponding pairs of input lists.</returns>
+    let zipShortest (l1: list<'T1>) (l2: list<'T2>) =
+        let rec loop acc = function
+            | (l::ls,r::rs) -> loop ((l,r)::acc) (ls,rs)
+            | (_,_) -> acc
+        loop [] (l1,l2) |> List.rev
+       
+            

src/FSharpPlus/Extensions/ResizeArray.fs

@@ -86,3 +86,17 @@ module ResizeArray =
         let (x, y) = ResizeArray (), ResizeArray ()
         Array.iter (mapper >> function Choice1Of2 e -> x.Add e | Choice2Of2 e -> y.Add e) source
         x.ToArray (), y.ToArray ()
+    
+    
+    /// <summary>
+    /// Zip safely two ResizeArrays. If one ResizeArray is shorter, excess elements are discarded from the right end of the longer ResizeArray. 
+    /// </summary>
+    /// <param name="a1">First input ResizeArray.</param>
+    /// <param name="a2">Second input ResizeArray.</param>
+    /// <returns>ResizeArray with corresponding pairs of input ResizeArrays.</returns>
+    let zipShortest (a1: ResizeArray<'T1>) (a2: ResizeArray<'T2>) =
+        let len = min a1.Count a2.Count
+        let ra = ResizeArray(len)
+        for i in 0..(len-1) do
+            ra.Add (a1.[i], a2.[i])
+        ra

src/FSharpPlus/Operators.fs

@@ -90,13 +90,14 @@ module Operators =
     /// Like map but ignoring the results.
     let inline iter (action: 'T->unit) (source: '``Functor<'T>``) : unit = Iterate.Invoke action source
 
-    // Un-zips (un-tuple) two functors.
+    /// Un-zips (un-tuple) two functors.
     let inline unzip (source: '``Functor<'T1 * 'T2>``) = Unzip.Invoke source : '``Functor<'T1>`` * '``Functor<'T2>``
 
-    // Zips (tuple) two functors.
+    /// Zips (tuple) two functors.
+    /// For collections, if one collection is shorter, excess elements are discarded from the right end of the longer collection. 
     let inline zip (source1: '``ZipFunctor<'T1>``) (source2: '``ZipFunctor<'T2>``) : '``ZipFunctor<'T1 * 'T2>`` = Zip.Invoke source1 source2
-   
 
+    
     // Applicative ------------------------------------------------------------
 
 

tests/FSharpPlus.Tests/Data.fs

@@ -352,6 +352,11 @@ module NonEmptyList =
     [<Test>]
     let ``zip `` () =
         nonEmptyList |> NonEmptyList.zip nonEmptyList |> NonEmptyList.toList |> shoulSeqEqual [(1,1)]
+    
+    [<Test>]
+    let zipShortest () =
+        let nonEmptyList' = nonEmptyList |> NonEmptyList.cons 2
+        nonEmptyList |> NonEmptyList.zipShortest nonEmptyList' |> NonEmptyList.toList |> shoulSeqEqual [(2,1)]
 
     [<Test>]
     let ``get head from NonEmptyList`` () =

tests/FSharpPlus.Tests/General.fs

@@ -489,6 +489,19 @@ module Functor =
 
         ()
 
+    [<Test>]
+    let genericZipShortest () =
+        let a = zip [|1; 2; 3|]  [|"a"; "b"|]
+        CollectionAssert.AreEqual ([|1,"a"; 2,"b"|], a)
+        
+        let l = zip [1; 2]  ["a"; "b"; "c"]
+        CollectionAssert.AreEqual ([1,"a"; 2,"b"], l)
+        
+        let e = zip (ResizeArray [1; 2]) (ResizeArray ["a"; "b"; "c"])
+        CollectionAssert.AreEqual (ResizeArray [1,"a"; 2,"b"], e)
+        
+        let nel = zip (NonEmptyList.ofList [1; 2]) (NonEmptyList.ofList ["a"; "b"; "c"])
+        CollectionAssert.AreEqual (NonEmptyList.ofList [1,"a"; 2,"b"], nel)
 
 module Collections =