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2014-12-20 Grepping from within Emacs

It is a well-known fact that a hacker’s life without grep(1) would be rather terrible. It is extremely useful. It is a bit less known that a hacker’s life without Emacs is rather terrible. However, given two great tools, one wonders whether they could not be made to work together?

Oh yes, they could. Starting grep from within Emacs is not only possible, but also advisible, for reasons I will explain in a minute.

A quick C-h a grep RET shows (among others) the commands grep-find, grep, lgrep and rgrep. (There are more, but we will deal with these basic ones only.)

The most basic of them is just grep. It shows a prompt saying grep -nH -e, which means that the only thing you have to do is to type the search pattern (a regex) and a list of filenames (possibly with wildcard(s)). What is more Emacs-y than just running M-! grep ... is that the results are shown in a buffer put in grep-mode, which inherits from compilation-mode. This means that you can do all sorts of cool stuff there – just press C-h m to see for yourself. Two highlights are: M-n and M-p for the next and previous match (respectively), C-c C-c (or just RET) to jump to the file and position shown at point, and (probably the coolest trick) C-c C-f to enable next-error-follow-minor-mode, which shows the match in another window dynamically, i.e., just moving point in the *grep* buffer causes Emacs to show the match in context.

Now the disadvantage to this is that you have to remember to give the pattern first and the filename(s) next (which is kind of obvious, but anyway), and – which is more cumbersome – to quote things like backslashes properly. Both these problems are addressed by lgrep, which asks for the pattern to search (conveniently deriving the default from the word at point), then the filename(s) (conveniently deriving the default from the current file’s extension) and finally the directory (conveniently deriving the default from the current dir). Prefixed with C-u it even allows to modify the actual command line before execution. All that is really useful.

What is a real pain in the neck, however, is grepping recursively. Yes, there is grep -r, but for more complicated searches a combination of find and grep is needed, and anyone knows that the only thing worse than the syntax of find(1) is the syntax of tar(1). Yes, Emacs has grep-find, which gives a template for a find/grep combo, but one yearns for something more. And something more there is indeed: rgrep. Just like lgrep is a wrapper around grep itself, rgrep is a wrapper around a find/grep engine, with similar features.

As I said, there is more than that, but this covers at leasts the basics of searching files from within Emacs. Happy grepping!

CategoryEnglish, CategoryBlog, CategoryEmacs

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2014-12-13 Escaping madness

I was writing a part of my Org-edu-HTML tool, and needed to generate a piece of JavaScript declaring a string variable. The string is given by the user and as such is assumed to contain things like Robert'); DROP TABLE Students;-- (well, not exactly that in this case, but if you pass a user-defined string to (lambda (string) (format "var crazyString='%s';" string)), you are asking for trouble. What needed to be done was (if I get all this stuff correctly) escaping <, >, &, but also the quotes: ', " (just in case I decided to change var crazyString = '...'; to var crazyString = "...";) and – importantly – the backslash. While writing the function to do this (which I based on org-html-encode-plain-text, which incidentally is a very nice example of functional programming), I wrote this funny-looking expression, which at a first glance does nothing:

(replace-regexp-in-string "\\\\" "\\\\" text t t)

It should be more or less clear what it does: in the former string, we have a regex consisting of two backslashes (since backslashes in string literals in Elisp have to be escaped); this way, this regex matches a single backslash (since backslashes in Elisp regexen have to be escaped). The latter string is literal (=not a regex), so it consists of two backslashes (again), which are then interpreted by JavaScript as one backslash (since backslashes in JS strings have to escaped).


CategoryEnglish, CategoryBlog, CategoryEmacs, CategoryOrgMode

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2014-12-06 Announcement – Org-edu-HTML

This is more or less a copy of a message on the Org-mode mailing list I posted there some time ago.

I started working on a custom exporter from Org to HTML for educational materials. My vision is that there will be (some kind) of syntax in Org (most probably, I’m going to (ab)use the existing syntax) for specifying various kinds of exercises.

The project page is on GitHub. It is currently (still) in a very early stage, when it should be considered more of a proof-of-concept rather than anything useful (though I am going to actually use it in a few days for a real project). What currently (sort of) works are: single and multiple choice tests and cloze tests.

The design goals are:

  • simplicity: users should be able to leverage Org’s syntax and editing to quickly create educational materials (most of what is needed is provided by Org itself – the only thing that is lacking is activities like choice tests, cloze tests and other types).
  • configurability: the result should be clearly separated into HTML (with all the content), JS/jQuery script responsible for executing tests (if possible, completely generic – all the content and markup, like which answer is correct, will be in the HTML), and CSS responsible for visual presentation. Currently, there is no CSS whatsoever, so it looks rather plain; I plan to create a default, non-appalling theme. (The problem is that I’m rather new to the CSS and JS/jQuery parts, so I’m learning while coding – this means that it’s neither a fast nor effective process. But I’m patient.)
    A side effect of putting everything in HTML is that C-u in the browser will reveal all the correct answers. I consider this a feature, not a bug, since the aim of the tests is not to grade, but teach. AFAIU the web (though I may well be mistaken), there is no way to overcome this problem with HTML and JS alone anyway.
  • two backends: I plan to create also a LaTeX variant, so that it would be easy for users to create printed copies for e.g. exams, with the choice tests, cloze tests etc. typeset nicely, with an option to include all of the answers (for the teacher’s copy). Again, the LaTeX exporter would do the bulk of the job.
  • hopefully, a SCORM exporter, so that the users will be able to put everything in an LMS of their choice. (This is the main reason I started hacking the org-one-to-many project, which is currently dormant – but I will come back to it one day.)

As of now, the project just has everything in one directory, together with the example Org file (jQuery v2.1.1 minified also needs to be present there, or the variable org-edu-html-jquery-address should be set accordingly). If anyone is interested in such a project, any feedback (like bug reports/feature suggestions) is welcome. I would especially like to hear about what syntax might be a good idea on the Org side. (My limited experience from this and one other project, where I use Org to prepare e-learning materials shows that Org-mode might, alas, be not the best choice for that, though it definitely has its advantages, too.) Currently, for the choice tests I abused lists with checkboxes (which mark the correct answers). Also, any comments on the quality of the code (both on the Elisp and jQuery sides) are more than welcome – while I feel quite confident writing (at least simple) Elisp functions (though have yet much to learn from more experienced hackers), my jQuery experience is next to none. And last but not least, I’m curious whether there is any demand for such a thing (I assume it’s possible, since many Org users come from academia).

CategoryEnglish, CategoryBlog, CategoryEmacs, CategoryOrgMode, CategoryLaTeX

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