Move toString to ConversionFunctions and document

fhirpath/src/main/java/au/csiro/pathling/fhirpath/function/ConversionFunctions.java

@@ -0,0 +1,42 @@
+package au.csiro.pathling.fhirpath.function;
+
+import au.csiro.pathling.fhirpath.StringCoercible;
+import au.csiro.pathling.fhirpath.collection.Collection;
+import au.csiro.pathling.fhirpath.validation.FhirPathFunction;
+import au.csiro.pathling.utilities.Preconditions;
+import javax.annotation.Nonnull;
+
+/**
+ * Functions for converting between different types.
+ *
+ * @see <a href="https://build.fhir.org/ig/HL7/FHIRPath/#conversion">FHIRPath Specification -
+ * Conversion</a>
+ */
+@SuppressWarnings("unused")
+public class ConversionFunctions {
+
+  /**
+   * If the input collection contains a single item, this function will return a single String if:
+   * <ul>
+   * <li>the item in the input collection is a String</li>
+   * <li>the item in the input collection is an Integer,
+   * Decimal, Date, Time, DateTime, or Quantity the output will contain its String representation</li>
+   * <li>the item is a Boolean, where true results in 'true' and false in 'false'.</li>
+   * </ul>
+   * If the item is not one of the above types, the result is false.
+   *
+   * @param input The input collection
+   * @return A collection containing the single item as a String, or an empty collection if the
+   * input collection is empty or contains more than one item.
+   * @see <a href="https://build.fhir.org/ig/HL7/FHIRPath/#tostring--string">FHIRPath Specification
+   * - toString</a>
+   */
+  @FhirPathFunction
+  @Nonnull
+  public static Collection toString(@Nonnull final Collection input) {
+    Preconditions.checkUserInput(input instanceof StringCoercible,
+        "toString() can only be applied to a StringCoercible path");
+    return ((StringCoercible) input).asStringPath();
+  }
+
+}

fhirpath/src/main/java/au/csiro/pathling/fhirpath/function/StandardFunctions.java

@@ -17,26 +17,12 @@
 
 package au.csiro.pathling.fhirpath.function;
 
-import au.csiro.pathling.fhirpath.StringCoercible;
-import au.csiro.pathling.fhirpath.collection.Collection;
-import au.csiro.pathling.fhirpath.validation.FhirPathFunction;
-import au.csiro.pathling.utilities.Preconditions;
-import javax.annotation.Nonnull;
-
 /**
  * Implementation of standard FHIRPath functions.
  */
 @SuppressWarnings("unused")
 public class StandardFunctions {
 
-  // TODO: This should be a string collection with a StringCoercible argument
-  @FhirPathFunction
-  public static Collection toString(@Nonnull final Collection input) {
-    Preconditions.checkUserInput(input instanceof StringCoercible,
-        "toString() can only be applied to a StringCoercible path");
-    return ((StringCoercible) input).asStringPath();
-  }
-
   private StandardFunctions() {
   }