Variable width fonts are generally more readable, which is why they’re generally used for prose.

Fixed-width fonts allow for easy tabular rendering in a text editor. Some programming styles use this heavily but it’s not universal. Particularly, if you rely on line-by-line blame in source control, alignment changes can make it harder to find the commit where the most recent semantic change happened.

Traditional fonts even had non-lining figures, with varying heights for easier readability. Modern-style figures were only really used for tabular display, or with content written in uppercase/title-case. (Of course, it's hard to support this kind of complex text styling when editing plain-text files that are going to be fed to a compiler, or some other text-processing step.)

But source code is very different from prose. Are there any studies on fixed-width vs. proportional readability specifically when it comes to programming?

I use Geneva for coding. Variable width typefaces are easier on my eyes. I find that my page-scanning more readily gets lulled into laziness when using fixed width. And variable width keeps me from doing goofy alignment games with blocks of similar text (e.g. lining up the `=` signs in a list of assignment operations) that I invariably regret later.

The kerning is easier in your eyes, and is also much more space efficient. The only drawback is the inability to do ascii art.

While I wouldn't say it's THAT significant, there is one other advantage to fixed-width fonts that I've experienced. Due to the fact that they align vertically, sometimes they expose mistakes if you're constructing data in a table-like fashion. Or when you have a bunch of copy/pasted lines that differ only in the parameter values. Like, "Hey wait a minute... this line is 1 char wider than the previous one. Oh, whoops, I fat-fingered something there".

I wish I had more specific examples and could quantify it better, but I know I've caught quite a few errors this way.

This exactly. Most recently I had a script that was calling functions with fixed width data sets (IDs, credit card numbers, birthdates, etc..), and typos were significantly easier to find due to the data and fonts both being fixed width.

This is because we don’t have any decent rich formatting options available for code in a plain text file. You could totally find those errors in an excel spreadsheet, and excel doesn’t default to using fixed-width typewriter fonts.

Maybe it's because I've only used fixed-width fonts for programming and have gotten used to it - but I find fixed-width fonts significantly easier on the eyes.

But would you ever read a novel using fixed width fonts? Modern typography uses variable width fonts for good reasons.

I find type writer fonts to look horrible since I’ve gotten used to variable width fonts. It’s like immediate eye strain for me. But you have to choose a good font, times new Roman won’t work.

I used DejaVu Mono Book for quite a while, I had no problems with it.

Only when coding or always, for example books and websites including this one?

Only when coding or working with raw text, I use variable-width fonts everywhere else (reading, browsing the web, etc).

And aligning text in some indentation styles

Stroustrup uses them in The C++ Programming Language and it is made it much nicer to read.

If you're using tabs for indentation, and not trying to do pointless alignment then there's really no reason to use monospaced fonts at all - it just makes things uglier.