add spec.optional

Author: atomicptr

spec.lua

@@ -66,6 +66,18 @@ function M.exists(value)
     return value ~= nil
 end
 
+---Tests if value is supplied that it matches the spec
+---@param spec fun(value: any): boolean
+---@return fun(value: any): boolean
+function M.optional(spec)
+    return function(value)
+        if value then
+            return spec(value)
+        end
+        return true
+    end
+end
+
 ---Tests if all of predicates are valid
 ---@param ... fun(value: any): boolean
 ---@return fun(value: any): boolean

spec_test.lua

@@ -43,6 +43,12 @@ describe("spec.lua", function()
         assert.True(spec.exists "Test")
     end)
 
+    it("spec.optional", function()
+        assert.True(spec.optional(spec.string) "Hello, World")
+        assert.True(spec.optional(spec.string)(nil))
+        assert.False(spec.optional(spec.string)(true))
+    end)
+
     it("spec.all_of", function()
         assert.False(spec.valid(spec.all_of(spec.string, spec.number), nil))
         assert.True(spec.valid(spec.all_of(spec.table, spec.some), { 1, 2 }))