make spec.assert also return value

Author: atomicptr

README.md

@@ -77,6 +77,9 @@ spec.conform(user_spec, bob) -- returns nil
 
 -- assert that the spec is valid otherwise throw an error
 spec.assert(spec.string, "Hello, World")
+
+-- assert also returns the value if its valid so you can use this to check things passed into functions
+promote_user(spec.assert(user_spec, alice))
 ```
 
 ## License

spec.lua

@@ -159,10 +159,13 @@ function M.conform(spec, value)
 end
 
 ---Asserts that a spec is valid
----@param spec any
----@param value any
+---@generic T
+---@param spec fun(value: any): boolean
+---@param value T
+---@return T
 function M.assert(spec, value)
     assert(M.valid(spec, value))
+    return value
 end
 
 return M

spec_test.lua

@@ -228,5 +228,8 @@ describe("spec.lua", function()
         assert.has_error(function()
             spec.assert(spec.string, 1337)
         end)
+
+        local x = spec.assert(spec.number, 42)
+        assert.equal(42, x)
     end)
 end)