Add nil safe operator

[zachbadgett]

Takes care of #147

Only thing this doesn't work on is when the env struct is predefined before compiling due to OpEqualString and OpEqualInt optimizations doing type casts.

Not completely happy with the way it's implemented, but it was the easiest to do without changing much core code.

[antonmedv]

Awesome work! 👏 I'll try to take a closes look at this before merging.

[guillaume-merindol-cko]

Any news on this PR? :)

[antonmedv]

Sorry for delay! @davidklassen can you also take a look? Thanks.

[davidklassen]

LGTM

[antonmedv]

Found an error. In this case error should be returned, but instead is returned.

package main

import (
	"fmt"

	"github.com/antonmedv/expr"
)

type S struct {
	S *S
	A string
}

func main() {
	env := map[string]interface{}{
		"s": S{S: &S{S: &S{A: "OK"}}},
	}

	code := `s?.S.NOPE.A`

	program, err := expr.Compile(code, expr.Env(env))
	if err != nil {
		panic(err)
	}

	output, err := expr.Run(program, env)
	if err != nil {
		panic(err)
	}

	fmt.Println(output)
}

Part with S.NOPE should throw error as not nil safe.

@zachbadgett can you take a look? Thanks.

[antonmedv]

The problem is with parser:

go run github.com/antonmedv/expr/cmd/exe -ast <<< 'a?.b.c'
PropertyNode{
	Node: PropertyNode{
		Node: IdentifierNode{
			Value: "a",
			NilSafe: true,
		},
		Property: "b",
-		NilSafe: true,
	},
	Property: "c",
-	NilSafe: true,
}

[antonmedv]

One year later, I think this is not error any more)))