lang: funcs: Add a weekday function to the datetime package

This returns the day of the week. It also includes a helpful example
demonstrating how this functionality can be fun!

examples/lang/readonlyfriday.mcl

@@ -0,0 +1,30 @@
+import "datetime"
+import "fmt"
+
+$now = datetime.now()
+$day = datetime.weekday($now)
+$is_friday = $day == "friday"
+
+$s1 = template("Hello! It is now: {{ datetime_print . }}\n", $now)
+$s2 = if $is_friday {
+	"It's friday!!! (don't break anything, read-only)"
+} else {
+	if $day == "saturday" || $day == "sunday" {
+		"It's the weekend!"
+	} else {
+		fmt.printf("Unfortunately, it is %s. Go to work!", $day)
+	}
+}
+
+print "msg" {
+	msg => $s1 + $s2,
+}
+
+file "/tmp/files/" {
+	state => "exists",
+	mode => if $is_friday { # this updates the mode, the instant it changes!
+		"0550"
+	} else {
+		"0770"
+	},
+}

lang/funcs/core/datetime/weekday_func.go

@@ -0,0 +1,49 @@
+// Mgmt
+// Copyright (C) 2013-2019+ James Shubin and the project contributors
+// Written by James Shubin <james@shubin.ca> 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 <http://www.gnu.org/licenses/>.
+
+package coredatetime
+
+import (
+	"fmt"
+	"strings"
+	"time"
+
+	"github.com/purpleidea/mgmt/lang/funcs/simple"
+	"github.com/purpleidea/mgmt/lang/types"
+)
+
+func init() {
+	simple.ModuleRegister(moduleName, "weekday", &types.FuncValue{
+		T: types.NewType("func(a int) str"),
+		V: Weekday,
+	})
+}
+
+// Weekday returns the lowercased day of the week corresponding to the input
+// time. The time is the number of seconds since the epoch, and matches what
+// comes from our Now function.
+func Weekday(input []types.Value) (types.Value, error) {
+	epochDelta := input[0].Int()
+	if epochDelta < 0 {
+		return nil, fmt.Errorf("epoch delta must be positive")
+	}
+
+	weekday := time.Unix(epochDelta, 0).Weekday()
+	return &types.StrValue{
+		V: strings.ToLower(weekday.String()),
+	}, nil
+}