Printf fixes around handling of -0.0 (negative zero)

docs/release-notes/.FSharp.Core/9.0.200.md

@@ -2,6 +2,7 @@
 
 * Fix exception on Post after MailboxProcessor was disposed ([Issue #17849](https://github.com/dotnet/fsharp/issues/17849), [PR #17922](https://github.com/dotnet/fsharp/pull/17922))
 * Fix missing null annotation in Async.SwitchToContext ([Issue #18055](https://github.com/dotnet/fsharp/issues/18055), [PR #18059](https://github.com/dotnet/fsharp/pull/18059))
+* Fix printf handling of -0.0 (negative zero) values for float, float32, and decimal values ([Issue #15557](https://github.com/dotnet/fsharp/issues/15557) and [Issue #15558](https://github.com/dotnet/fsharp/issues/15558), [PR #18147](https://github.com/dotnet/fsharp/pull/18147))
 
 ### Added
 

src/FSharp.Core/printf.fs

@@ -658,8 +658,12 @@ module internal PrintfImpl =
 
     /// Contains functions to handle left/right and no justification case for numbers
     module GenericNumber =
-
-        let isPositive (n: obj) = 
+
+        let inline singleIsPositive n = n >= 0.0f
+        let inline doubleIsPositive n = n >= 0.0
+        let inline decimalIsPositive n = n >= 0.0M
+
+        let isPositive (n: obj) =
             match n with 
             | :? int8 as n -> n >= 0y
             | :? uint8 -> true
@@ -671,9 +675,9 @@ module internal PrintfImpl =
             | :? uint64 -> true
             | :? nativeint as n -> n >= 0n
             | :? unativeint -> true
-            | :? single as n -> n >= 0.0f
-            | :? double as n -> n >= 0.0
-            | :? decimal as n -> n >= 0.0M
+            | :? single as n -> singleIsPositive n
+            | :? double as n -> doubleIsPositive n
+            | :? decimal as n -> decimalIsPositive n
             | _ -> failwith "isPositive: unreachable"
 
         /// handles right justification when pad char = '0'
@@ -851,12 +855,24 @@ module internal PrintfImpl =
             | _ -> invalidArg (nameof spec) "Invalid integer format"
 
     module FloatAndDecimal = 
-
+
+        let fixupSign isPositive (nStr: string) =
+            // .NET Core and .NET Framework differ in how ToString and other formatting methods handle certain negative floating-point values (namely, -0.0 and values which round to -0.0 upon display).
+            // (see: https://devblogs.microsoft.com/dotnet/floating-point-parsing-and-formatting-improvements-in-net-core-3-0/)
+            // So in order for F#'s sprintf to behave consistently across platforms, we essentially "polyfill" (normalize) the output to ToString across the two runtimes. Specifically we do this by
+            // removing the '-' character in situations where the rest of the sprintf logic treats the number as positive, but .NET Core treats it as negative (i.e. -0.0, or -0.0000000001 when
+            // displaying with only a few decimal places)
+            // TODO: make this work for numbers like -0.0000000001
+            if isPositive && nStr.StartsWith "-" then
+                nStr.Substring 1
+            else
+                nStr
+
         let rec toFormattedString fmt (v: obj) = 
             match v with
-            | :? single as n -> n.ToString(fmt, CultureInfo.InvariantCulture)
-            | :? double as n -> n.ToString(fmt, CultureInfo.InvariantCulture)
-            | :? decimal as n -> n.ToString(fmt, CultureInfo.InvariantCulture)
+            | :? single as n -> n.ToString(fmt, CultureInfo.InvariantCulture) |> fixupSign (GenericNumber.singleIsPositive n)
+            | :? double as n -> n.ToString(fmt, CultureInfo.InvariantCulture) |> fixupSign (GenericNumber.doubleIsPositive n)
+            | :? decimal as n -> n.ToString(fmt, CultureInfo.InvariantCulture) |> fixupSign (GenericNumber.decimalIsPositive n)
             | _ -> failwith "toFormattedString: unreachable"
 
         let isNumber (x: obj) =

tests/FSharp.Core.UnitTests/FSharp.Core/Microsoft.FSharp.Core/PrintfTests.fs

@@ -75,6 +75,102 @@ type PrintfTests() =
         Assert.AreEqual("      7B", sprintf "%*X"  8 123  )
         Assert.AreEqual("7B      ", sprintf "%-*X" 8 123  ) 
 
+    // test cases for https://github.com/dotnet/fsharp/issues/15557
+    [<Fact>]
+    member this.``sign flag - positive and negative one``() =
+        test "%f"  +1.0         "1.000000"
+        test "%f"  -1.0         "-1.000000"
+        test "%+f" +1.0         "+1.000000"
+        test "%+f" -1.0         "-1.000000"
+
+        test "%f"  +1.0f        "1.000000"
+        test "%f"  -1.0f        "-1.000000"
+        test "%+f" +1.0f        "+1.000000"
+        test "%+f" -1.0f        "-1.000000"
+
+        test "%f"  +1.0M        "1.000000"
+        test "%f"  -1.0M        "-1.000000"
+        test "%+f" +1.0M        "+1.000000"
+        test "%+f" -1.0M        "-1.000000"
+
+    [<Fact>]
+    member this.``sign flag - positive and negative zero``() =
+        test "%f"  +0.0         "0.000000"
+        test "%f"  -0.0         "0.000000"
+        test "%+f" +0.0         "+0.000000"
+        test "%+f" -0.0         "+0.000000"
+
+        test "%f"  +0.0f        "0.000000"
+        test "%f"  -0.0f        "0.000000"
+        test "%+f" +0.0f        "+0.000000"
+        test "%+f" -0.0f        "+0.000000"
+
+        test "%f"  +0.0M        "0.000000"
+        test "%f"  -0.0M        "0.000000"
+        test "%+f" +0.0M        "+0.000000"
+        test "%+f" -0.0M        "+0.000000"
+
+    [<Fact>]
+    member this.``sign flag - very small positive and negative numbers``() =
+        test "%f"  -0.0000001   "0.000000"
+        // TODO: should this output -0.000000 or +0.000000? See https://github.com/dotnet/fsharp/pull/18147#issuecomment-2546220183
+        // test "%+f" -0.0000001   "+0.000000"
+
+        test "%f"  -0.0000001f  "0.000000"
+        // see previous comment
+        // test "%+f" -0.0000001f  "+0.000000"
+
+        test "%f"  -0.0000001M  "0.000000"
+        // see previous comment
+        // test "%+f" -0.0000001M  "+0.000000"
+
+    [<Fact>]
+    member this.``sign flag - infinity``() =
+        test "%f"  +infinity    "Infinity"
+        test "%f"  -infinity    "-Infinity"
+        test "%+f" +infinity    "Infinity"
+        test "%+f" -infinity    "-Infinity"
+
+        test "%f"  +infinityf   "Infinity"
+        test "%f"  -infinityf   "-Infinity"
+        test "%+f" +infinityf   "Infinity"
+        test "%+f" -infinityf   "-Infinity"
+
+    [<Fact>]
+    member this.``sign flag - NaN``() =
+        test "%f"  +nan         "NaN"
+        test "%f"  -nan         "NaN"
+        test "%+f" +nan         "NaN"
+        test "%+f" -nan         "NaN"
+
+        test "%f"  +nanf        "NaN"
+        test "%f"  -nanf        "NaN"
+        test "%+f" +nanf        "NaN"
+        test "%+f" -nanf        "NaN"
+
+    // test cases for https://github.com/dotnet/fsharp/issues/15558 (same root cause as #15557; listing for completeness)
+    [<Fact>]
+    member this.``zero padding - positive and negative one`` () =
+        test "%010.3f" +1.0  "000001.000"
+        test "%010.3f" -1.0  "-00001.000"
+
+        test "%010.3f" +1.0f "000001.000"
+        test "%010.3f" -1.0f "-00001.000"
+
+        test "%010.3f" +1.0M "000001.000"
+        test "%010.3f" -1.0M "-00001.000"
+
+    [<Fact>]
+    member this.``zero padding - positive and negative zero`` () =
+        test "%010.3f" +0.0         "000000.000"
+        test "%010.3f" -0.0         "000000.000"
+
+        test "%010.3f" +0.0f        "000000.000"
+        test "%010.3f" -0.0f        "000000.000"
+
+        test "%010.3f" +0.0M        "000000.000"
+        test "%010.3f" -0.0M        "000000.000"
+
     [<Fact>]
     member _.``union case formatting`` () =
         Assert.AreEqual("CaseOne", sprintf "%A" CaseOne)