#2451 throw exception when sequence is empty

Author: Michał Niegrzybowski

src/fable-library/Seq.fs

@@ -807,13 +807,17 @@ let average (xs: seq<'T>) ([<Inject>] averager: IGenericAverager<'T>): 'T =
     let mutable count = 0
     let folder acc x = count <- count + 1; averager.Add(acc, x)
     let total = fold folder (averager.GetZero()) xs
-    averager.DivideByInt(total, count)
+    if count = 0 then
+        invalidArg "xs" LanguagePrimitives.ErrorStrings.InputSequenceEmptyString
+    else averager.DivideByInt(total, count)
 
 let averageBy (f: 'T -> 'U) (xs: seq<'T>) ([<Inject>] averager: IGenericAverager<'U>): 'U =
     let mutable count = 0
     let inline folder acc x = count <- count + 1; averager.Add(acc, f x)
     let total = fold folder (averager.GetZero()) xs
-    averager.DivideByInt(total, count)
+    if count = 0 then
+        invalidArg "xs" LanguagePrimitives.ErrorStrings.InputSequenceEmptyString
+    else averager.DivideByInt(total, count)
 
 let permute f (xs: seq<'T>) =
     delay (fun () ->

tests/Main/SeqTests.fs

@@ -80,6 +80,14 @@ let tests =
         sumFirstTwo zs
         |> equal 1.
 
+    testCase "Seq.average for empty sequence" <| fun () ->
+        let xs = Seq.empty<float>
+        (try Seq.average xs |> ignore; false with | _ -> true) |> equal true
+
+    testCase "Seq.averageBy for empty sequence" <| fun () ->
+        let xs = Seq.empty<float>
+        (try Seq.averageBy ((*) 2.) xs |> ignore; false with | _ -> true) |> equal true
+
     testCase "Seq.average works" <| fun () ->
         let xs = seq {1.; 2.; 3.; 4.}
         Seq.average xs