import(
    "fmt"
    "log"

    "github.com/google/cel-go/cel"
    "github.com/google/cel-go/checker/decls"
)

func main() {
    // Variables used within this expression environment.
    decls := cel.Declarations(
        decls.NewIdent("i", decls.String, nil),
        decls.NewIdent("you", decls.String, nil))
    env, err := cel.NewEnv(decls)
    if err != nil {
        log.Fatalf("environment creation error: %s\n", err)
    }

    // Parse and type-check the expression.
    //
    // Detailed information about which issues/errors were encountered may be
    // found within the `iss`, but a non-nil `iss.Err()` value will contain a
    // detailed error string in human-readable form.
    p, iss := env.Parse(`"Hello " + you + "! I'm " + i + "."`)
    if iss != nil && iss.Err() != nil {
        log.Fatalln(iss.Err())
    }
    c, iss := env.Check(p)
    if iss != nil && iss.Err() != nil {
        log.Fatalln(iss.Err())
    }

    // Create the program, and evaluate it against some input.
    prg, err := env.Program(c)
    if err != nil {
        log.Fatalf("program creation error: %s\n", err)
    }

    // Note, the second result, the EvalDetails will be non-nil when using
    // either cel.OptTrackState or cel.OptExhaustiveEval (not shown). The
    // details value holds information about the evaluation, rather than the
    // output of the evaluation itself.
    out, _, err := prg.Eval(cel.Vars(map[string]interface{}{
        "i": "CEL",
        "you": "world"}))
    if err != nil {
        log.Fatalf("runtime error: %s\n", err)
    }

    // Hello world! I'm CEL.
    fmt.Println(out)
}