lang: types, lang: core: strings: Add a new join_nonempty function

Along with a helper function for when we need a []string from a Value.

lang/core/strings/join_nonempty.go

@@ -0,0 +1,70 @@
+// Mgmt
+// Copyright (C) 2013-2024+ James Shubin and the project contributors
+// Written by James Shubin <[email protected]> and the project contributors
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+//
+// Additional permission under GNU GPL version 3 section 7
+//
+// If you modify this program, or any covered work, by linking or combining it
+// with embedded mcl code and modules (and that the embedded mcl code and
+// modules which link with this program, contain a copy of their source code in
+// the authoritative form) containing parts covered by the terms of any other
+// license, the licensors of this program grant you additional permission to
+// convey the resulting work. Furthermore, the licensors of this program grant
+// the original author, James Shubin, additional permission to update this
+// additional permission if he deems it necessary to achieve the goals of this
+// additional permission.
+
+package corestrings
+
+import (
+	"context"
+	"strings"
+
+	"github.com/purpleidea/mgmt/lang/funcs/simple"
+	"github.com/purpleidea/mgmt/lang/types"
+)
+
+func init() {
+	simple.ModuleRegister(ModuleName, "join_nonempty", &simple.Scaffold{
+		T: types.NewType("func(s []str, sep str) str"),
+		F: JoinNonempty,
+	})
+}
+
+// JoinNonempty behaves exactly like the regular string join, except it filters
+// out empty elements first. Since this behaviour is commonly useful when
+// writing scripts, we include it as a built-in primitive, since it is also
+// cheaper than running the graph changing, filter function.
+// TODO: compiler optimization: find patterns where someone used the filter
+// function as previously described, and replace it with this implementation.
+func JoinNonempty(ctx context.Context, input []types.Value) (types.Value, error) {
+	return &types.StrValue{
+		V: joinNonempty(types.ValueToListStr(input[0]), input[1].Str()),
+	}, nil
+}
+
+// joinNonempty is the actual implementation.
+func joinNonempty(elems []string, sep string) string {
+	l := []string{}
+	for _, x := range elems {
+		if x == "" {
+			continue
+		}
+		l = append(l, x)
+	}
+
+	return strings.Join(l, sep)
+}

lang/types/util.go

@@ -229,3 +229,15 @@ func ListStrToValue(input []string) Value {
 	}
 	return l
 }
+
+// ValueToListStr is a simple helper function to convert from a list of strings
+// in our type system to the equivalent in golang. This panics if the input is
+// not of the correct type.
+func ValueToListStr(input Value) []string {
+	l := []string{}
+	for _, x := range input.List() {
+		s := x.Str()
+		l = append(l, s)
+	}
+	return l
+}