adding some test cases and tweaking implementation to address them

lang/funcs/collection.go

@@ -113,7 +113,7 @@ var CoalesceListFunc = function.New(&function.Spec{
 	Params: []function.Parameter{},
 	VarParam: &function.Parameter{
 		Name:             "vals",
-		Type:             cty.DynamicPseudoType,
+		Type:             cty.List(cty.DynamicPseudoType),
 		AllowUnknown:     true,
 		AllowDynamicType: true,
 		AllowNull:        true,
@@ -170,7 +170,6 @@ var CompactFunc = function.New(&function.Spec{
 	},
 	Type: function.StaticReturnType(cty.List(cty.String)),
 	Impl: func(args []cty.Value, retType cty.Type) (ret cty.Value, err error) {
-
 		var outputList []cty.Value
 
 		for it := args[0].ElementIterator(); it.Next(); {
@@ -180,6 +179,11 @@ var CompactFunc = function.New(&function.Spec{
 			}
 			outputList = append(outputList, v)
 		}
+
+		if len(outputList) == 0 {
+			return cty.ListValEmpty(cty.String), nil
+		}
+
 		return cty.ListVal(outputList), nil
 	},
 })
@@ -223,6 +227,10 @@ var IndexFunc = function.New(&function.Spec{
 	},
 	Type: function.StaticReturnType(cty.Number),
 	Impl: func(args []cty.Value, retType cty.Type) (ret cty.Value, err error) {
+		if !(args[0].Type().IsListType() || args[0].Type().IsTupleType()) {
+			return cty.NilVal, fmt.Errorf("argument must be a list or tuple")
+		}
+
 		if args[0].LengthInt() == 0 { // Easy path
 			return cty.NilVal, fmt.Errorf("Cannot search an empty list")
 		}

lang/funcs/collection_test.go

@@ -299,6 +299,16 @@ func TestCoalesceList(t *testing.T) {
 			}),
 			false,
 		},
+		{
+			[]cty.Value{
+				cty.MapValEmpty(cty.DynamicPseudoType),
+				cty.ListVal([]cty.Value{
+					cty.StringVal("third"), cty.StringVal("fourth"),
+				}),
+			},
+			cty.NilVal,
+			true,
+		},
 	}
 
 	for _, test := range tests {
@@ -339,6 +349,20 @@ func TestCompact(t *testing.T) {
 			}),
 			false,
 		},
+		{
+			cty.ListVal([]cty.Value{
+				cty.StringVal(""),
+				cty.StringVal(""),
+				cty.StringVal(""),
+			}),
+			cty.ListValEmpty(cty.String),
+			false,
+		},
+		{
+			cty.ListValEmpty(cty.String),
+			cty.ListValEmpty(cty.String),
+			false,
+		},
 		{
 			cty.ListVal([]cty.Value{
 				cty.StringVal("test"),