DDevelopment & Design

Grammar parser in C++

Recently I stumbled upon implementing a simple parser in C++. The task is very classic, however, I couldn’t find any good resources on the web to help me out.
I tried different tools (including ANTLR), but finally, the easiest way I found was bison + flex. It’s unbelievable that this technology from 1989 is still actively developed. The latest stable release is from May 14, 2011. Moreover, many essential projects make use of it. Among them are Ruby, PHP, Google Go, and Bash shell.
So I decided to create a minimalistic example that works from scratch.
I published the code on GitHub Calculator, so you can check it out.
The whole example is 88 lines long and evaluates common expressions, like 2+2*2-13*(7+19/2).
Let’s start with lexer. In flex, you need to define regular expressions, which produce tokens. Such tokens are later processed by a scanner. So we have to define calculator.lex, like this:
Flex will generate yylex() function, which we can call later to produce tokens. Next, we need to create a scanner (calculator.y), which specifies a grammar. It’s simple like that:
Here, we specify types for all tokens using C/C++ union like structure. Variable $$ is used to store result of particular reductions.
Additionally, we need to specify %left precedence for +, -, *, / operators to resolve shift / reduce conflicts between them.
And that’s basically it. We have a working expression parser.
I implemented it so that executable takes a file name containing expressions as an argument.
So you can try ./calculator input.txt to see the result.

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DDevelopment & Design

Grails render as JSON catch

One of a reasons your controller doesn't render a proper response in JSON format might be wrong package name that you use. It is easy to overlook. Import are on top of a file, you look at your code and everything seems to be fine. Except response is still not in JSON format.

Consider this simple controller:

class RestJsonCatchController {
def grailsJson() {
render([first: 'foo', second: 5] as grails.converters.JSON)
    }

def netSfJson() {
render([first: 'foo', second: 5] as net.sf.json.JSON)
    }
}

And now, with finger crossed... We have a winner!

$ curl localhost:8080/example/restJsonCatch/grailsJson
{"first":"foo","second":5}
$ curl localhost:8080/example/restJsonCatch/netSfJson
{first=foo, second=5}

As you can see only grails.converters.JSON converts your response to JSON format. There is no such converter for net.sf.json.JSON, so Grails has no converter to apply and it renders Map normally.

Conclusion: always carefully look at your imports if you're working with JSON in Grails!

Edit: Burt suggested that this is a bug. I've submitted JIRA issue here: GRAILS-9622 render as class that is not a codec should throw exception