make the merge function more precise

lang/funcs/collection.go

@@ -865,35 +865,120 @@ var MatchkeysFunc = function.New(&function.Spec{
 	},
 })
 
-// MergeFunc constructs a function that takes an arbitrary number of maps and
-// returns a single map that contains a merged set of elements from all of the maps.
+// MergeFunc constructs a function that takes an arbitrary number of maps or objects, and
+// returns a single value that contains a merged set of keys and values from
+// all of the inputs.
 //
-// If more than one given map defines the same key then the one that is later in
-// the argument sequence takes precedence.
+// If more than one given map or object defines the same key then the one that
+// is later in the argument sequence takes precedence.
 var MergeFunc = function.New(&function.Spec{
 	Params: []function.Parameter{},
 	VarParam: &function.Parameter{
 		Name:             "maps",
 		Type:             cty.DynamicPseudoType,
 		AllowDynamicType: true,
+		AllowNull:        true,
+	},
+	Type: func(args []cty.Value) (cty.Type, error) {
+		// empty args is accepted, so assume an empty object since we have no
+		// key-value types.
+		if len(args) == 0 {
+			return cty.EmptyObject, nil
+		}
+
+		// collect the possible object attrs
+		attrs := map[string]cty.Type{}
+
+		first := cty.NilType
+		matching := true
+		attrsKnown := true
+		for i, arg := range args {
+			ty := arg.Type()
+			// any dynamic args mean we can't compute a type
+			if ty.Equals(cty.DynamicPseudoType) {
+				return cty.DynamicPseudoType, nil
+			}
+
+			// check for invalid arguments
+			if !ty.IsMapType() && !ty.IsObjectType() {
+				return cty.NilType, fmt.Errorf("arguments must be maps or objects, got %#v", ty.FriendlyName())
+			}
+
+			switch {
+			case ty.IsObjectType() && !arg.IsNull():
+				for attr, aty := range ty.AttributeTypes() {
+					attrs[attr] = aty
+				}
+			case ty.IsMapType():
+				switch {
+				case arg.IsNull():
+					// pass, nothing to add
+				case arg.IsKnown():
+					ety := arg.Type().ElementType()
+					for it := arg.ElementIterator(); it.Next(); {
+						attr, _ := it.Element()
+						attrs[attr.AsString()] = ety
+					}
+				default:
+					// any unknown maps means we don't know all possible attrs
+					// for the return type
+					attrsKnown = false
+				}
+			}
+
+			// record the first argument type for comparison
+			if i == 0 {
+				first = arg.Type()
+				continue
+			}
+
+			if !ty.Equals(first) && matching {
+				matching = false
+			}
+		}
+
+		// the types all match, so use the first argument type
+		if matching {
+			return first, nil
+		}
+
+		// We had a mix of unknown maps and objects, so we can't predict the
+		// attributes
+		if !attrsKnown {
+			return cty.DynamicPseudoType, nil
+		}
+
+		return cty.Object(attrs), nil
 	},
-	Type: function.StaticReturnType(cty.DynamicPseudoType),
 	Impl: func(args []cty.Value, retType cty.Type) (ret cty.Value, err error) {
 		outputMap := make(map[string]cty.Value)
 
+		// if all inputs are null, return a null value rather than an object
+		// with null attributes
+		allNull := true
 		for _, arg := range args {
-			if !arg.IsWhollyKnown() {
-				return cty.UnknownVal(retType), nil
-			}
-			if !arg.Type().IsObjectType() && !arg.Type().IsMapType() {
-				return cty.NilVal, fmt.Errorf("arguments must be maps or objects, got %#v", arg.Type().FriendlyName())
+			if arg.IsNull() {
+				continue
+			} else {
+				allNull = false
 			}
+
 			for it := arg.ElementIterator(); it.Next(); {
 				k, v := it.Element()
 				outputMap[k.AsString()] = v
 			}
 		}
-		return cty.ObjectVal(outputMap), nil
+
+		switch {
+		case allNull:
+			return cty.NullVal(retType), nil
+		case retType.IsMapType():
+			return cty.MapVal(outputMap), nil
+		case retType.IsObjectType(), retType.Equals(cty.DynamicPseudoType):
+			return cty.ObjectVal(outputMap), nil
+		default:
+			panic(fmt.Sprintf("unexpected return type: %#v", retType))
+		}
 	},
 })
 

lang/funcs/collection_test.go

@@ -2079,7 +2079,7 @@ func TestMerge(t *testing.T) {
 					"c": cty.StringVal("d"),
 				}),
 			},
-			cty.ObjectVal(map[string]cty.Value{
+			cty.MapVal(map[string]cty.Value{
 				"a": cty.StringVal("b"),
 				"c": cty.StringVal("d"),
 			}),
@@ -2094,6 +2094,65 @@ func TestMerge(t *testing.T) {
 					"c": cty.StringVal("d"),
 				}),
 			},
+			cty.MapVal(map[string]cty.Value{
+				"a": cty.UnknownVal(cty.String),
+				"c": cty.StringVal("d"),
+			}),
+			false,
+		},
+		{ // handle null map
+			[]cty.Value{
+				cty.NullVal(cty.Map(cty.String)),
+				cty.MapVal(map[string]cty.Value{
+					"c": cty.StringVal("d"),
+				}),
+			},
+			cty.MapVal(map[string]cty.Value{
+				"c": cty.StringVal("d"),
+			}),
+			false,
+		},
+		{ // handle null map
+			[]cty.Value{
+				cty.NullVal(cty.Map(cty.String)),
+				cty.NullVal(cty.Object(map[string]cty.Type{
+					"a": cty.List(cty.String),
+				})),
+			},
+			cty.NullVal(cty.EmptyObject),
+			false,
+		},
+		{ // handle null object
+			[]cty.Value{
+				cty.MapVal(map[string]cty.Value{
+					"c": cty.StringVal("d"),
+				}),
+				cty.NullVal(cty.Object(map[string]cty.Type{
+					"a": cty.List(cty.String),
+				})),
+			},
+			cty.ObjectVal(map[string]cty.Value{
+				"c": cty.StringVal("d"),
+			}),
+			false,
+		},
+		{ // handle unknowns
+			[]cty.Value{
+				cty.UnknownVal(cty.Map(cty.String)),
+				cty.MapVal(map[string]cty.Value{
+					"c": cty.StringVal("d"),
+				}),
+			},
+			cty.UnknownVal(cty.Map(cty.String)),
+			false,
+		},
+		{ // handle dynamic unknown
+			[]cty.Value{
+				cty.UnknownVal(cty.DynamicPseudoType),
+				cty.MapVal(map[string]cty.Value{
+					"c": cty.StringVal("d"),
+				}),
+			},
 			cty.DynamicVal,
 			false,
 		},
@@ -2107,7 +2166,7 @@ func TestMerge(t *testing.T) {
 					"a": cty.StringVal("x"),
 				}),
 			},
-			cty.ObjectVal(map[string]cty.Value{
+			cty.MapVal(map[string]cty.Value{
 				"a": cty.StringVal("x"),
 				"c": cty.StringVal("d"),
 			}),
@@ -2151,7 +2210,7 @@ func TestMerge(t *testing.T) {
 					}),
 				}),
 			},
-			cty.ObjectVal(map[string]cty.Value{
+			cty.MapVal(map[string]cty.Value{
 				"a": cty.MapVal(map[string]cty.Value{
 					"b": cty.StringVal("c"),
 				}),
@@ -2176,7 +2235,7 @@ func TestMerge(t *testing.T) {
 					}),
 				}),
 			},
-			cty.ObjectVal(map[string]cty.Value{
+			cty.MapVal(map[string]cty.Value{
 				"a": cty.ListVal([]cty.Value{
 					cty.StringVal("b"),
 					cty.StringVal("c"),
@@ -2213,6 +2272,66 @@ func TestMerge(t *testing.T) {
 			}),
 			false,
 		},
+		{ // merge objects of various shapes
+			[]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{
+					"a": cty.ListVal([]cty.Value{
+						cty.StringVal("b"),
+					}),
+				}),
+				cty.ObjectVal(map[string]cty.Value{
+					"d": cty.DynamicVal,
+				}),
+			},
+			cty.ObjectVal(map[string]cty.Value{
+				"a": cty.ListVal([]cty.Value{
+					cty.StringVal("b"),
+				}),
+				"d": cty.DynamicVal,
+			}),
+			false,
+		},
+		{ // merge maps and objects
+			[]cty.Value{
+				cty.MapVal(map[string]cty.Value{
+					"a": cty.ListVal([]cty.Value{
+						cty.StringVal("b"),
+					}),
+				}),
+				cty.ObjectVal(map[string]cty.Value{
+					"d": cty.NumberIntVal(2),
+				}),
+			},
+			cty.ObjectVal(map[string]cty.Value{
+				"a": cty.ListVal([]cty.Value{
+					cty.StringVal("b"),
+				}),
+				"d": cty.NumberIntVal(2),
+			}),
+			false,
+		},
+		{ // attr a type and value is overridden
+			[]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{
+					"a": cty.ListVal([]cty.Value{
+						cty.StringVal("b"),
+					}),
+					"b": cty.StringVal("b"),
+				}),
+				cty.ObjectVal(map[string]cty.Value{
+					"a": cty.ObjectVal(map[string]cty.Value{
+						"e": cty.StringVal("f"),
+					}),
+				}),
+			},
+			cty.ObjectVal(map[string]cty.Value{
+				"a": cty.ObjectVal(map[string]cty.Value{
+					"e": cty.StringVal("f"),
+				}),
+				"b": cty.StringVal("b"),
+			}),
+			false,
+		},
 		{ // argument error: non map type
 			[]cty.Value{
 				cty.MapVal(map[string]cty.Value{

website/docs/configuration/functions/merge.html.md

@@ -3,8 +3,9 @@ layout: "functions"
 page_title: "merge - Functions - Configuration Language"
 sidebar_current: "docs-funcs-collection-merge"
 description: |-
-  The merge function takes an arbitrary number of maps and returns a single
-  map after merging the keys from each argument.
+  The merge function takes an arbitrary number maps or objects, and returns a
+  single map or object that contains a merged set of elements from all
+  arguments.
 ---
 
 # `merge` Function
@@ -13,19 +14,33 @@ description: |-
 earlier, see
 [0.11 Configuration Language: Interpolation Syntax](../../configuration-0-11/interpolation.html).
 
-`merge` takes an arbitrary number of maps and returns a single map that
-contains a merged set of elements from all of the maps.
+`merge` takes an arbitrary number of maps or objects, and returns a single map
+pr object that contains a merged set of elements from all arguments. 
 
-If more than one given map defines the same key then the one that is later
-in the argument sequence takes precedence.
+If more than one given map or object defines the same key or attribute, then
+the one that is later in the argument sequence takes precedence. If the
+argument types do not match, the resulting type will be an object matching the
+type structure of the attributes after the merging rules have been applied.
 
 ## Examples
 
 ```
-> merge({"a"="b", "c"="d"}, {"e"="f", "c"="z"})
+> merge({a="b", c="d"}, {e="f", c="z"})
 {
   "a" = "b"
   "c" = "z"
   "e" = "f"
 }
 ```
+
+```
+> merge({a="b"}, {a=[1,2], c="z"}, {d=3})
+{
+  "a" = [
+    1,
+    2,
+  ]
+  "c" = "z"
+  "d" = 3
+}
+```