Refactor parsePrefixExpression, block some expressions #189

[u5surf]

resolved: #189

details: I added validation rules to block prefix ‘!‘ is not valid for StringLiteral.
and ‘-’ is also invalid for Boolean.

[task detail...]

1. I added validations to block invalid prefix for some Literals.
2. I fix the TestMakePrefix

• Test case
  I fix TestMakePrefix to pass invalid error meesage to indicate blocking following [parser] refactor parsePrefixExpression, block some expressions #189 cases.

[zeroFruit]

First of all, thank you for contributing to our project! @u5surf 👍

Feedback

The logic is ok. But I think we can refactor and improve more.

1. How about validating operation and right expression first, then create whole expression?
   The old code create whole expression first then filling the operation and right expression. I think after validating op and right expression then creates whole expression is more make sense.

func parsePrefixExpression(buf TokenBuffer) (ast.Expression, error) {
	tok := buf.Read()

	op := operatorMap[tok.Type]
	right, err := parseExpression(buf, PREFIX)

        // validating logic

	// create whole expression
}

2. We can use switch instead of if. I think using switch can be more readable and scalable.

switch op {
case ast.Bang:
    ...
case ast.Minus:
    ...
}

and

switch right.(type) {
case *ast.BooleanLiteral:
    ...
case *ast.StringLiteral:    
    ...
}

[junbeomlee]

Error has occurred !!

parse/parser.go:381:16: too few values in parseError literal
parse/parser.go:389:16: too few values in parseError literal

plz fix the error
you can check this in here https://travis-ci.org/DE-labtory/koa/builds/483891010?utm_source=github_status&utm_medium=notification

[junbeomlee]

There is still an error in the code.
Could u check parse/parser.go: 381 lines and
389 line!

[u5surf]

Oh, I got it, parseError has added some element imperceptibly!

[zeroFruit]

I think this err is not needed