Special processing for printing ResizeArray<_> in Property.forAll #323

src/Hedgehog/Property.fs

@@ -81,11 +81,28 @@ module Property =
     let bind (k : 'a -> Property<'b>) (m : Property<'a>) : Property<'b> =
         bindGen (toGen << k) (toGen m) |> ofGen
 
+    let private printValue (value) : string =
+        // sprintf "%A" is not prepared for printing ResizeArray<_> (C# List<T>) so we prepare the value instead
+        let prepareForPrinting (value: obj) : obj =
+        #if FABLE_COMPILER
+            value
+        #else
+            let t = value.GetType()
+            // have to use TypeInfo due to targeting netstandard 1.6
+            let t = System.Reflection.IntrospectionExtensions.GetTypeInfo(t)
+            let isList = t.IsGenericType && t.GetGenericTypeDefinition() = typedefof<ResizeArray<_>>
+            if isList
+            then value :?> System.Collections.IEnumerable |> Seq.cast<obj> |> List.ofSeq :> obj
+            else value
+        #endif
+
+        value |> prepareForPrinting |> sprintf "%A"
+
     let forAll (k : 'a -> Property<'b>) (gen : Gen<'a>) : Property<'b> =
         let handle (e : exn) =
             Gen.constant (Journal.singletonMessage (string e), Failure) |> ofGen
         let prepend (x : 'a) =
-            counterexample (fun () -> sprintf "%A" x)
+            counterexample (fun () -> printValue x)
             |> bind (fun _ -> try k x with e -> handle e)
             |> toGen
 

tests/Hedgehog.Tests/Hedgehog.Tests.fsproj

@@ -16,6 +16,7 @@
     <Compile Include="SeedTests.fs" />
     <Compile Include="ShrinkTests.fs" />
     <Compile Include="MinimalTests.fs" />
+    <Compile Include="PropertyTests.fs" />
     <Compile Include="Program.fs" />
   </ItemGroup>
   <ItemGroup>

tests/Hedgehog.Tests/Program.fs

@@ -13,6 +13,7 @@ let allTests = testList "All tests" [
     SeedTests.seedTests
     ShrinkTests.shrinkTests
     MinimalTests.minimalTests
+    PropertyTests.propertyTests
 ]
 
 [<EntryPoint>]

tests/Hedgehog.Tests/PropertyTests.fs

@@ -0,0 +1,17 @@
+module Hedgehog.Tests.PropertyTests
+
+open Hedgehog
+open Expecto
+open TestDsl
+
+let propertyTests = testList "Property tests" [
+    fableIgnore "generated C# list of five elements is not abbreviated in the failure report" <| fun _ ->
+        let report =
+            property {
+                let! xs = Range.singleton 0 |> Gen.int |> Gen.list (Range.singleton 5) |> Gen.map ResizeArray
+                return false
+            }
+            |> Property.renderWith (PropertyConfig.withShrinks 0<shrinks> PropertyConfig.defaultConfig)
+        Expect.isNotMatch report "\.\.\." "Abbreviation (...) found"
+
+]