Fix numeric formatting patterns by unwrapping scalar values (#3135)

Author: sisve

src/kOS.Safe.Test/Structures/StringValueTest.cs

@@ -219,5 +219,16 @@ public void CanParseScientificStrings()
             stringTest = new StringValue(" 1.23e+3a ");
             Assert.Throws(typeof(Exceptions.KOSNumberParseException), () => stringTest.ToScalar());
         }
+
+        [Test]
+        public void CanFormat()
+        {
+            var formatString = new StringValue("test1={0:0.0} test2='{0,10:0.0}'");
+
+            var expected = new StringValue("test1=13.4 test2='      13.4'");
+            var actual = formatString.Format(new ScalarDoubleValue(13.37));
+
+            Assert.That(expected.ToString(), Is.EqualTo(actual.ToString()));
+        }
     }
 }

src/kOS.Safe/Encapsulation/StringValue.cs

@@ -7,6 +7,7 @@
 using kOS.Safe.Serialization;
 using System.Collections.Generic;
 using System.Collections;
+using System.Linq;
 
 namespace kOS.Safe.Encapsulation
 {
@@ -257,7 +258,21 @@ public StringValue Format(params Structure[] args)
         {
             if (args.Length == 0)
                 return this;
-            return new StringValue(string.Format(CultureInfo.InvariantCulture, this, args));
+
+            // Unwrap the primitive scalar value and send them
+            // into String.Format as-is. This is required to allow
+            // for numeric formatting patterns, like rounding.
+            var primitiveArgs = args
+                .Select(arg => {
+                    if (arg is ScalarValue scalar) {
+                        return scalar.ToPrimitive();
+                    }
+
+                    return arg;
+                })
+                .ToArray();
+
+            return new StringValue(string.Format(CultureInfo.InvariantCulture, this, primitiveArgs));
         }
 
         private void StringInitializeSuffixes()