Prototype: using Go's ast tools to build a comment parsing contract engine.

README.md

Overview

$ go run law.go
package main

import "fmt"

// @require: not-empty: len(x) > 0
func Foo(x string) {
        if !(len(x) > 0) {
                panic("not-empty")
        }

        fmt.Printf("Hello, %s!\n", x)
}

func main() {
        Foo("")
}

Idea: go test supports -cover which will rewrite your source code, annotating it with line coverage statements that provide a way in which to report code coverage. A new tool that could augment the build and/or test tools for the purposes of enforcing contracts during debug, or testing would be really neat. Beyond this basic prototype, however, I'm not yet sure how to get there...

law.go

package main

import (
	"fmt"
	"go/ast"
	"go/parser"
	"go/printer"
	"go/token"
	"os"
	"strings"
)

const Program = `
package main

import "fmt"

// @require: not-empty: len(x) > 0
func Foo(x string) {
  fmt.Printf("Hello, %s!\n", x)
}

func main() {
  Foo("")
}
`

func main() {
	fset := token.NewFileSet()
	f, err := parser.ParseFile(fset, "foo.go", []byte(Program), parser.ParseComments)
	if err != nil {
		fmt.Println(err)
		return
	}

	for _, dec := range f.Decls {
		if fdec, ok := dec.(*ast.FuncDecl); ok {
			if fdec.Doc != nil {
				for _, c := range fdec.Doc.List {
					bits := strings.Split(c.Text, ":")
					if len(bits) != 3 {
						continue
					}
					if !strings.Contains(bits[0], "@require") {
						continue
					}

					contractName := fmt.Sprintf("%q", strings.TrimSpace(bits[1]))
					contractRawExpr := bits[2]

					expr, err := parser.ParseExprFrom(fset, "foo.go", []byte(contractRawExpr), 0)
					if err != nil {
						fmt.Printf("Unable to parse contract expression: %s", err)
						os.Exit(1)
					}

					ifs := &ast.IfStmt{
						Cond: &ast.UnaryExpr{Op: token.NOT, X: expr},
						Body: &ast.BlockStmt{
							List: []ast.Stmt{
								&ast.ExprStmt{
									X: &ast.CallExpr{
										Fun:  ast.NewIdent("panic"),
										Args: []ast.Expr{&ast.BasicLit{Kind: token.STRING, Value: contractName}},
									},
								},
							},
						},
					}

					newBody := []ast.Stmt{ifs}
					fdec.Body.List = append(newBody, fdec.Body.List...)
				}
			}
		}
	}

	printer.Fprint(os.Stdout, fset, f)
}