MetaEdit+ - Quick MERL concatenating in MetaEdit+ 5.0

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Quick MERL concatenating in MetaEdit+ 5.0

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Forum Name: MetaEdit+
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URL: https://www.metacase.com/forums/forum_posts.asp?TID=538
Printed Date: 27.Mar.2026 at 00:26
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Topic: Quick MERL concatenating in MetaEdit+ 5.0

Posted By: stevek
Subject: Quick MERL concatenating in MetaEdit+ 5.0
Date Posted: 13.Sep.2012 at 14:52

In MERL we can assign variables:

$a = 'foo'
$b = $a

but we can't assign multiple strings in a single assignment:

$c = 'foo' $a

That gets parsed as an assignment $c ='foo', plus an output of $a. In 4.5 for an assignment and a concatenation, you used the full variable command:

variable 'c' write
'foo'
$a
close

In 5.0 we can use subgenerators as functions, with parentheses surrounding the comma-separated arguments. Each argument can be formed by the output of multiple commands: just like in Java you can write "foo(name+" "+surname, b)", in MERL you can write "foo($name ' ' $surname, $b)". MERL concatenates the output from all of an argument's commands, then passes the result as the argument.

So if we define a MERL generator function that simply outputs its single argument:

concat(@arg)
@arg

we can use it to concatenate multiple commands. This lets us rewrite our initial example as:

$c = concat('foo' $a) /* NB only one argument, not concat ('foo', $a) */

Or even better, if we rename concat() to __():

$c = __('foo' $a)

which will be ignored because of its __ prefix when building the generator call tree hierarchy display, and so become almost part of the language. And now in all the places where before you could only have a single command (assignment RHS, comparison LHS and RHS...), you can have a concatenation almost for free.