+ tryItem fallback overload matching on exn

Author: gusty

src/FSharpPlus/Control/Indexable.fs

@@ -16,8 +16,10 @@ open FSharpPlus.Internals.MonadOps
 
 type Item =
     inherit Default1
+
+    static member inline InvokeOnInstance (k: 'K) (x: '``Indexable<'T>``) : 'T = (^``Indexable<'T>`` : (member get_Item : _ -> 'T) x, k) : 'T
 
-    static member inline Item (x: '``Indexable<'T>`` , k        , [<Optional>]_impl: Default1) = (^``Indexable<'T>`` : (member get_Item : _ -> 'T) x, k) : 'T
+    static member inline Item (x: '``Indexable<'T>`` , k        , [<Optional>]_impl: Default1) = Item.InvokeOnInstance k x : 'T
     static member inline Item (_: 'T when 'T: null and 'T: struct, _,         _impl: Default1) = ()
 
     static member        Item (x: string             , n        , [<Optional>]_impl: Item    ) = String.item n x
@@ -37,6 +39,12 @@ type Item =
 
 type TryItem =
     inherit Default1
+
+    static member inline TryItem (x: '``Indexable<'T>``, k, [<Optional>]_impl: Default3) =
+        try Some (Item.InvokeOnInstance k x) with
+        | :? ArgumentException -> None 
+        | _ -> reraise ()
+
     static member inline TryItem (x: '``Indexable<'T>``, k, [<Optional>]_impl: Default2) =
         let mutable r = Unchecked.defaultof< ^R>
         if (^``Indexable<'T>``: (member TryGetValue: _ * _ -> _) (x, k, &r)) then Some r else None

tests/FSharpPlus.Tests/Indexables.fs

@@ -13,6 +13,34 @@ open Helpers
 
 module Indexables =
 
+    type NdXable1 = NdXable1 with static member    TryItem (x, NdXable1) = if x = 1 then Some "Item retrieved" else None
+    type NdXable3 = NdXable3 with        member _.get_Item  x            = if x = 1 then      "Item retrieved" else invalidArg "item" (string x)
+
+    [<Test>]
+    let testTryItem () =
+        let a1 = tryItem 0 NdXable1
+        Assert.AreEqual (None, a1)
+        let b1 = tryItem 1 NdXable1
+        Assert.AreEqual (Some "Item retrieved", b1)
+
+        let dct = dict [1, "one"; 2, "two"]
+        let a2 = tryItem 0 dct
+        Assert.AreEqual (None, a2)
+        let b2 = tryItem 1 dct
+        Assert.AreEqual (Some "one", b2)
+
+        let a3 = tryItem 0 NdXable3
+        Assert.AreEqual (None, a3)
+        let b3 = tryItem 1 NdXable3
+        Assert.AreEqual (Some "Item retrieved", b3)
+
+        let lst = ["zero"; "one"; "two"]
+        let a4 = tryItem 10 lst
+        Assert.AreEqual (None, a4)
+        let b4 = tryItem 1 lst
+        Assert.AreEqual (Some "one", b4)
+
+
     [<Test>]
     let testCompileAndExecuteItem () =
 
@@ -53,6 +81,7 @@ module Indexables =
         // let f = seq [1, "one"; 2, "two"]
         // let _ = item 1 f
 
+
         ()
 
     [<Test>]