DDevelopment & Design

Turing completeness

  • bypawelz
  • October 23, 2010
  • 2 minute read

sed is a very powerful tool. A simple sed statement may turn a cat into cement. Observe: echo cat | sed statement I was asked by mariom@ircnet whether is it possible to implement the Euklidean algorithm (the one that computes the greatest common divisor) in awk. awk is a Turing complete language, so the answer is yes, it is possible. There is a snippet proposed by mariom:

{
    a = $1;
    b = $2;
    while (b != 0) {
        c = a % b;
        a = b;
        b = c;
    }
    print a
}

Anyway, my response was that it is possible even in sed. Is it? Of course! Sed is a Turing complete language. Though I had no idea how to write it in sed. I use sed for simple substitutions only, but I could not admit that! I started googling for arithmetic operations in sed and I found

a dc implementation in pure sed. So the next question is how to implement the Euklidean algorithm in dc. It turned out to be quite simple, see:

[lalb%sclbsalcsblb0

This code assumes that there are two integers on the stack. So you can test it with something like that:

echo '20 25 [lalb%sclbsalcsblb0

Let’s analyze what is happening there. There is F macro that is equivalent of “while” loop in awk code. It

loads the values of a and b registers on the stack. Then replaces them with their reminder and saves it in the c registers.

lbsalcsb

statement just copies the value of

b to a and c to b. Finaly it compares the value of b with 0. If former is greater it executes F (note: recurrence. It is the only way to implement a loop in dc). Otherwise it quits.

sasblFxlap

statement is an entry point. It saves values from stack in the 

a and b registers, then executes F and prints the content of the a register. So this sed script is an equivalent of awk script that executes Euklidean algothm. Now it is enough to embed the dc script into dc.sed, et voila! Enjoy: Euclidean algorithm implemented in pure sed.

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Simple trick to DRY your Grails controller

Grails controllers are not very DRY. It's easy to find duplicated code fragments in default generated controller. Take a look at code sample below. It is duplicated four times in show, edit, update and delete actions:

class BookController {
def show() {
def bookInstance = Book.get(params.id)
if (!bookInstance) {
flash.message = message(code: 'default.not.found.message', args: [message(code: 'book.label', default: 'Book'), params.id])
redirect(action: "list")
return
        }
        [bookInstance: bookInstance]
    }
}

Why is it duplicated?

There is a reason for that duplication, though. If you move this snippet to a method, it can redirect to "list" action, but it can't prevent controller from further execution. After you call redirect, response status changes to 302, but after method exits, controller still runs subsequent code.

Solution

At TouK we've implemented a simple trick to resolve that situation:

  1. wrap everything with a simple withStoppingOnRender method,
  2. whenever you want to render or redirect AND stop controller execution - throw EndRenderingException.

We call it Big Return - return from a method and return from a controller at once. Here is how it works:

class BookController {
def show(Long id) {
withStoppingOnRender {
Book bookInstance = Book.get(id)
validateInstanceExists(bookInstance)
            [bookInstance: bookInstance]
        }
    }

protected Object withStoppingOnRender(Closure closure) {
try {
return closure.call()
        } catch (EndRenderingException e) {}
    }

private void validateInstanceExists(Book instance) {
if (!instance) {
flash.message = message(code: 'default.not.found.message', args: [message(code: 'book.label', default: 'Book'), params.id])
redirect(action: "list")
throw new EndRenderingException()
        }
    }
}

class EndRenderingException extends RuntimeException {}

Example usage

For simple CRUD controllers, you can use this solution and create some BaseController class for your controllers. We use withStoppingOnRender in every controller so code doesn't look like a spaghetti, we follow DRY principle and code is self-documented. Win-win-win! Here is a more complex example:

class DealerController {
@Transactional
def update() {
withStoppingOnRender {
Dealer dealerInstance = Dealer.get(params.id)
validateInstanceExists(dealerInstance)
validateAccountInExternalService(dealerInstance)
checkIfInstanceWasConcurrentlyModified(dealerInstance, params.version)
            dealerInstance.properties = params
saveUpdatedInstance(dealerInstance)
redirectToAfterUpdate(dealerInstance)
        }
    }
}