Fix https://github.com/DiffSharp/DiffSharp/issues/27

src/DiffSharp/Symbolic.Float32.fs

@@ -60,7 +60,7 @@ module ExprOps =
         | SpecificCall <@ (-) @> (_, _, [a; b]) -> let at, bt = diffExpr v a, diffExpr v b in <@@ (%%at:float32) - %%bt @@>
         | SpecificCall <@ (*) @> (_, _, [a; b]) -> let at, bt = diffExpr v a, diffExpr v b in <@@ (%%at:float32) * %%b + %%a * %%bt @@>
         | SpecificCall <@ (/) @> (_, _, [a; b]) -> let at, bt = diffExpr v a, diffExpr v b in <@@ ((%%at:float32) * %%b - %%a * %%bt) / (%%b * %%b)@@>
-        | SpecificCall <@ op_Exponentiation @> (_, _, [a; b]) -> let at, bt = diffExpr v a, diffExpr v b in <@@ ((%%a:float32) ** %%b) * ((%%b * %%at / %%a) + ((log %%a) * %%bt)) @@>
+        | SpecificCall <@ op_Exponentiation @> (_, _, [a; b]) -> let at, bt = diffExpr v a, diffExpr v b in <@@ if (%%a <> 0.0f) then ((%%a:float32) ** %%b) * ((%%b * %%at / %%a) + ((log %%a) * %%bt)) else 0.0f @@>
         | SpecificCall <@ atan2 @> (_, _, [a; b]) -> let at, bt = diffExpr v a, diffExpr v b in <@@ ((%%at:float32) * %%b - %%a * %%bt) / (%%a * %%a + %%b * %%b) @@>
         | SpecificCall <@ (~-) @> (_, _, [a]) -> let at = diffExpr v a in <@@ -(%%at:float32) @@>
         | SpecificCall <@ log @> (_, _, [a]) -> let at = diffExpr v a in <@@ (%%at:float32) / %%a @@>

src/DiffSharp/Symbolic.Float64.fs

@@ -60,7 +60,7 @@ module ExprOps =
         | SpecificCall <@ (-) @> (_, _, [a; b]) -> let at, bt = diffExpr v a, diffExpr v b in <@@ (%%at:float) - %%bt @@>
         | SpecificCall <@ (*) @> (_, _, [a; b]) -> let at, bt = diffExpr v a, diffExpr v b in <@@ (%%at:float) * %%b + %%a * %%bt @@>
         | SpecificCall <@ (/) @> (_, _, [a; b]) -> let at, bt = diffExpr v a, diffExpr v b in <@@ ((%%at:float) * %%b - %%a * %%bt) / (%%b * %%b)@@>
-        | SpecificCall <@ op_Exponentiation @> (_, _, [a; b]) -> let at, bt = diffExpr v a, diffExpr v b in <@@ ((%%a:float) ** %%b) * ((%%b * %%at / %%a) + ((log %%a) * %%bt)) @@>
+        | SpecificCall <@ op_Exponentiation @> (_, _, [a; b]) -> let at, bt = diffExpr v a, diffExpr v b in <@@ if (%%a <> 0.0) then ((%%a:float) ** %%b) * ((%%b * %%at / %%a) + ((log %%a) * %%bt)) else 0.0 @@>
         | SpecificCall <@ atan2 @> (_, _, [a; b]) -> let at, bt = diffExpr v a, diffExpr v b in <@@ ((%%at:float) * %%b - %%a * %%bt) / (%%a * %%a + %%b * %%b) @@>
         | SpecificCall <@ (~-) @> (_, _, [a]) -> let at = diffExpr v a in <@@ -(%%at:float) @@>
         | SpecificCall <@ log @> (_, _, [a]) -> let at = diffExpr v a in <@@ (%%at:float) / %%a @@>

tests/DiffSharp.Tests/DiffSharp.Tests.fsproj

@@ -61,6 +61,7 @@
     <Compile Include="Tests.fs" />
     <Compile Include="Backend.OpenBLAS.fs" />
     <Compile Include="AD.Float32.fs" />
+    <Compile Include="Symbolic.fs" />
     <None Include="Script.fsx" />
     <Content Include="paket.references" />
     <Content Include="app.config" />

tests/DiffSharp.Tests/Symbolic.fs

@@ -0,0 +1,41 @@
+module Symbolic
+
+open FsCheck.NUnit
+open DiffSharp.Util
+open DiffSharp.Tests
+open NUnit.Framework
+
+module Float64 = 
+    open DiffSharp.Symbolic.Float64
+    [<ReflectedDefinition>]
+    let f2 x = x**2.0
+
+    [<Test>]
+    // See https://github.com/DiffSharp/DiffSharp/issues/27
+    let ``Diff.Of.Exponential.At.Zero``() = 
+        let dy = diff <@ f2 @>
+        Assert.True( Util.(=~) (dy 0.0, 0.0))
+
+    [<Test>]
+    let ``Diff.Of.Exponential.At.One``() = 
+        let dy = diff <@ f2 @>
+        Assert.True(Util.(=~) (dy 1.0, 2.0))
+
+module Float32 = 
+    open DiffSharp.Symbolic.Float32
+
+    [<ReflectedDefinition>]
+    let f2 x = x**2.0f
+
+    [<Test>]
+    // See https://github.com/DiffSharp/DiffSharp/issues/27
+    let ``Diff.Of.Exponential.At.Zero``() = 
+        printfn "testing"
+        let dy = diff <@ f2 @>
+        Assert.True(Util.(=~) (dy 0.0f, 0.0f))
+
+    [<Test>]
+    let ``Diff.Of.Exponential.At.One``() = 
+        let dy = diff <@ f2 @>
+        Assert.True(Util.(=~) (dy 1.0f, 2.0f))
+