SubX

The subx package provide a (test) matcher library for use with Go 1.18beta1 or newer, relying on type parametrization. As Go 1.18 isn't yet released, this code is naturally unsuitable for production.

Fundamentals

This library revolve around three main concepts:

1: A "check" or CheckFunc, is a function that takes a value initializer function and returns a well formatted error if the initialized value does not pass the check:

type CheckFunc[T any] func(func() T) error

2: A "value initializer" or func () T, is any function that return a single parameter. Because the same value function can be passed to multiple checks, it is expected to return a stable result.

3: Finally, the generic Test[T] function converts a value initializer and a compatible check into a test function that can be passed to t.Run. Note that thanks to the generic design's type constraints, this is a type-safe operation. Thanks to the type inference system, the code is still easy to read.

Examples:

vf := func() int {
	return 8
}

// Valid:
t.Run("Expect correct result", subx.Test(vf, subx.OrderGreater(0))
t.Run("Expect correct result", subx.Test(vf, subx.AllOf(
	subx.OrderGreater(0),
	subx.OrderLess(10),
)))

// Invalid:
t.Run("Expect correct result", subx.Test(vf, subx.AllOf(
	subx.OrderGreater(0),
	subx.OrderLess(10),
	subx.StringHasPrefix("F"), // Compiler error; trying to use a CheckFunc[string].
)))

vf := func() string {
	return "FOO"
}

// Valid:
t.Run("Expect correct result", subx.Test(vf, subx.AllOf(
	subx.OrderGreater("A"),
	subx.OrderLess("Z"),
	subx.StringHasPrefix("F"),
)))

// Invalid:
t.Run("Expect correct result", subx.Test(vf, subx.AllOf(
	subx.OrderGreater(0), // Compiler error; trying to use a CheckFunc[int].
	subx.OrderLess('Z'),  // Compiler error; trying to use a CheckFunc[char].
	subx.StringHasPrefix("F"),
)))

Getting started with Go 1.18

Install go 1.18beta1 with tooling:

go install golang.org/dl/go1.18beta1@latest
go1.18beta1 download
go1.18beta1 install golang.org/x/tools/gopls@latest

If you are using VS Code, you can configure it to use Go 1.18beta1 by running "Go: Choose Go Environment" in the command menu (CTRL+P or CMD+P).

Git clone and test subx

Next up, git clone subx into the folder of your choosing.

git clone git@github.com:smyrman/subx

To test that everything is working, you can now try to run one of the example tests:

$ cd subx/examples/incorrect_sum
$ go1.18beta1 test
--- FAIL: TestSum (0.00s)
    --- FAIL: TestSum/[int](2,2,1) (0.00s)
        --- FAIL: TestSum/[int](2,2,1)/Expect_correct_and_positive_result (0.00s)
            sum_test.go2:16: 1/2 checks failed, want all checks to pass;
                [0]: comparison failed:
                    got: (int)
                        0
                    want equal to: (int)
                        5
    --- FAIL: TestSum/[float64](2,-3) (0.00s)
        --- FAIL: TestSum/[float64](2,-3)/Expect_correct_result (0.00s)
            sum_test.go2:29: comparison failed:
                got: (float64)
                    0
                want equal to: (float64)
                    -1
FAIL
exit status 1
FAIL	github.com/smyrman/subx/examples/incorrect_sum	0.358s

Set up your own project

To create your own project using generics, you can now use go modules more or less as you normally would

mkdir <project>
cd <project>
go1.18beta1 mod init github.com/<username>/<project>
go1.18beta1 get github.com/smyrman/subx@main

Make sure the go.mod file says go 1.18.