prep

prep is a small Go tool that enables compile-time function evaluation. By using prep.Comptime, you can evaluate functions at build time, replacing them with their computed results. Just like comptime from Zig. Except it's not.

Features

• Compile-Time Evaluation: Replace function calls with their computed results at build time.
• Simple Integration: Use prep as both a Go library and a standalone executable.
• Tooling Support: Easily integrate prep with your Go build process using -toolexec.

Installation

1. Install the prep executable

To use prep in your build process, install it as a binary executable:

go install github.com/pijng/prep/cmd/prep@latest

2. Add prep as a dependency

To use the prep library in your Go project, add it via Go modules:

go get github.com/pijng/prep
go mod tidy

Usage

Wrapping Functions with prep.Comptime

To mark a function for compile-time evaluation, wrap it with prep.Comptime:

package main

import (
  "fmt"
  "github.com/pijng/prep"
)

func main() {
  // This will be evaluated at compile-time
  result := prep.Comptime(fibonacci(300))

  fmt.Println("Result:", result)
}

func fibonacci(n int) int {
  fmt.Printf("calculating fibonacci for %d\n", n)

  if n <= 1 {
    return n
  }
  a, b := 0, 1
  for i := 2; i <= n; i++ {
    a, b = b, a+b
  }
  return b
}

Building with prep

After wrapping your functions with prep.Comptime, you need to use prep during the build process. This is done by using the -toolexec flag:

go build -toolexec="prep" main.go

This command will evaluate all functions wrapped with prep.Comptime and replace them with their computed results during the build.

Run the final binary:

./main

Note the speed and absence of calculating fibonacci for %d message.

Limitations

Basic Literal Arguments

At it's core, prep.Comptime allows you to provide basic literals (e.g., strings, integers, floats) as arguments. Which means you can either:

1. Pass a basic literal directly like this:

func job() {
  prep.Comptime(myFunc(1))
}

2. Use a variable with the value of basic literal from the same scope as wrapped function:

func job() {
  x := 1
  y := 2
  prep.Comptime(myFunc(x, y))
}

These literals are evaluated directly at compile-time

Compile-Time Evaluation (Experimental)

Additionally, prep.Comptime also supports evaluation during compilation when you can use values from IO operations, for example:

func main() {
  config := prep.Comptime(readFile("./test.txt"))
  fmt.Println(config)
}

func readFile(path string) string {
  content, _ := os.ReadFile(path)

  return string(content)
}

This way the prep will call readFile function as part of compilation step and include it's result to the final binary. This means you can remove the test.txt file after compilation, and still be able to execute the binary as if it exists.

Note that this feature is somewhat experimental and prone to errors.

Motivation

Because reasons.