prelude_bool.mk: Add

prelude.mk

@@ -8,6 +8,11 @@
 #  (none)
 ## Outputs ##
 #
+#  Boolean support:
+#  - Variable: TRUE  = T
+#  - Variable: FALSE =
+#  - Function: not
+#
 #  String support:
 #  - Variable: export NL
 #  - Variable:        SPACE
@@ -53,6 +58,7 @@ _prelude.mk := $(lastword $(MAKEFILE_LIST))
 # [GMSL](https://gmsl.sourceforge.io/).  Absolutely nothing in any of
 # the `prelude_*.mk` files is allowed to be eager, so ordering doesn't
 # matter.  Anything eager must go in this main `prelude.mk` file.
+include $(dir $(_prelude.mk))prelude_bool.mk
 include $(dir $(_prelude.mk))prelude_str.mk
 include $(dir $(_prelude.mk))prelude_path.mk
 include $(dir $(_prelude.mk))prelude_go.mk

prelude_bool.mk

@@ -0,0 +1,12 @@
+# This is part of `prelude.mk`, split out for organizational purposes.
+# !!! NOTHING EAGER IS ALLOWED TO HAPPEN IN THIS FILE !!!
+
+#
+# Boolean support
+
+# $(TRUE) is a non-empty string; $(FALSE) is an empty string.
+TRUE  = T
+FALSE =
+
+# Usage: $(call not,BOOL)
+not = $(if $1,$(FALSE),$(TRUE))

tests/prelude_bool.bats

@@ -0,0 +1,23 @@
+#!/usr/bin/env bats
+
+load common
+
+@test "prelude_bool.mk: TRUE" {
+	check_expr_eq strict '$(if $(TRUE),pass,fail)' 'pass'
+}
+
+@test "prelude_bool.mk: FALSE" {
+	check_expr_eq strict '$(if $(FALSE),fail,pass)' 'pass'
+}
+
+@test "prelude_bool.mk: not TRUE" {
+	check_expr_eq strict '$(if $(call not,$(TRUE)),fail,pass)' 'pass'
+}
+
+@test "prelude_bool.mk: not FALSE" {
+	check_expr_eq strict '$(if $(call not,$(FALSE)),pass,fail)' 'pass'
+}
+
+@test "prelude_bool.mk: not truthy" {
+	check_expr_eq strict '$(if $(call not,truthy),fail,pass)' 'pass'
+}