Avoid unnecessary list use in productGen.

Also expose sequenceToArr and sequenceToSeq to F# users.

Author: kurtschelfthout

src/FsCheck/Gen.fs

@@ -336,7 +336,7 @@ module Gen =
         map (fun (_, y, _) -> y) g,
         map (fun (_, _, z) -> z) g
 
-    ///Sequence the given seq of generators into a generator of a list.
+    /// Sequence the given enumerable of generators into a generator of a list.
     //[category: Creating generators from generators]
     [<CompiledName("SequenceToList"); EditorBrowsable(EditorBrowsableState.Never)>]
     let sequence l = 
@@ -349,9 +349,9 @@ module Gen =
                 go gs' (y::acc) size r2
         Gen(fun n r -> go (Seq.toList l) [] n r)
 
-    ///Sequence the given list of generators into a generator of a list.
+    /// Sequence the given enumerable of generators into a generator of an enumerable.
     //[category: Creating generators from generators]
-    [<CompiledName("Sequence"); CompilerMessage("This method is not intended for use from F#.", 10001, IsHidden=true, IsError=false)>]
+    [<CompiledName("Sequence")>]
     let sequenceToSeq generators = 
         // This implementation is similar to that for arrays and lists but is specialized
         // to sequences to avoid intermediate conversion to an intermediate list.
@@ -364,9 +364,9 @@ module Gen =
                 yield g size r1
             }
 
-    ///Sequence the given array of generators into a generator of a array.
+    /// Sequence the given array of generators into a generator of a array.
     //[category: Creating generators from generators]
-    [<CompiledName("Sequence"); CompilerMessage("This method is not intended for use from F#.", 10001, IsHidden=true, IsError=false)>]
+    [<CompiledName("Sequence")>]
     let sequenceToArr ([<ParamArrayAttribute>]generators:array<Gen<_>>) = 
         if Object.ReferenceEquals (null, generators) then
             nullArg "generators"

src/FsCheck/ReflectArbitrary.fs

@@ -58,13 +58,13 @@ module internal ReflectArbitrary =
                 fields |> Seq.exists (fun field -> isRecursive field containingType newSeen))
 
         let productGen (ts : seq<Type>) create =
-            let gs = [ for t in ts -> getGenerator t ]
+            let gs = [| for t in ts -> getGenerator t |]
             let n = gs.Length
-            if n = 0 then
+            if n <= 0 then
                 Gen.constant (create [||])
             else
-                sized (fun s -> resize ((s / n) - 1) (sequence gs))
-                |> map (List.toArray >> create)
+                sized (fun s -> resize (max 0 ((s / n) - 1)) (sequenceToArr gs))
+                |> map create
 
         if isRecordType t then
             let fields = getRecordFieldTypes t