Add less_than function utility module

init.lua

@@ -40,6 +40,7 @@ for _, file in pairs{
 	"schema",
 	"file",
 	"func",
+	"less_than",
 	"iterator",
 	"math",
 	"table",

less_than.lua

@@ -0,0 +1,53 @@
+-- Comparator utilities for "less than" functions returning whether a < b
+local less_than = {}
+setfenv(1, less_than)
+
+default = {}
+
+function default.less_than(a, b) return a < b end; default.lt = default.less_than
+function default.less_or_equal(a, b) return a <= b end; default.leq = default.less_or_equal
+function default.greater_than(a, b) return a > b end; default.gt = default.greater_than
+function default.greater_or_equal(a, b) return a >= b end; default.geq = default.greater_or_equal
+
+function less_or_equal(less_than)
+	return function(a, b) return not less_than(b, a) end
+end
+leq = less_or_equal
+
+function greater_or_equal(less_than)
+	return function(a, b) return not less_than(a, b) end
+end
+geq = greater_or_equal
+
+function greater_than(less_than)
+	return function(a, b) return less_than(b, a) end
+end
+gt = greater_than
+
+function equal(less_than)
+	return function(a, b)
+		return not (less_than(a, b) or less_than(b, a))
+	end
+end
+
+function relation(less_than)
+	return function(a, b)
+		if less_than(a, b) then return "<"
+		elseif less_than(b, a) then return ">"
+		else return "=" end
+	end
+end
+
+function by_func(func)
+	return function(a, b)
+		return func(a) < func(b)
+	end
+end
+
+function by_field(key)
+	return function(a, b)
+		return a[key] < b[key]
+	end
+end
+
+return less_than