Please scroll

:: Racket, Emacs

Recently I got more time to catch up on racket-mode. I improved two things that happen to fit one theme — an extraordinarily advanced UX concept I call, “scrolling down to the point of interest.”

Visit definition

In racket-mode you can M-. (press Alt and period) to visit the definition of a Racket identifier.

There is no Racket library function that supports this, exactly. The identifier-binding function gives you a filename, but not a position within. And actually, it gives you two filenames (and identifier symbols), because the location and symbol of the definition might differ from the location and symbol under which it is provided.

I’ve also found it can be tricky when something is renamed more than once — for example both renamed and wrapped in a contract. Of the three (or more) names involved, identifier-binding will return only two. For example in (provide (contract-out [rename orig new contract])) it reports (1) the contract wrapper’s generated identifier and (2) new — but not (3) orig. Unfortunately the definition of orig is our desired destination.

So, I need to treat identifier-binding as a valuable head start — but maybe not the real answer.

I need to check each file and try to find the identifier within. This isn’t a job for regular expression; the name might not appear textually at the definition site (think of define-er macros). Instead I read the file as syntax and walk it. Sometimes it makes sense to search the syntax after it has been fully-expanded. Sometimes it helps to walk it unexpanded, looking for some special forms, for example the “rename” variant of contract-out.

If after all that, we can’t find the position, racket-mode plops you at the start of the file.

The change I made was to reduce the chance of that happening. Details in the commit message and diff.

View documentation

In racket-mode you can C-c C-d to view Racket’s HTML documentation in your default web browser. It should (a) open the correct page and (b) scroll to the item on that page. Unfortunately (b) didn’t always happen on macOS. Under certain conditions, macOS is reluctant to open file: URLs and scroll down to the anchor/fragment (the bit after the # in the URL).

# Will open the default browser to the top of the define.html page
# but not scroll down to the define-values item:
$ open 'file:///Applications/Racket_v6.7/doc/reference/define.html#%28form._%28%28quote._~23~25kernel%29._define-values%29%29'

# Ditto
$ osascript -e 'open location "file:///Applications/Racket_v6.7/doc/reference/define.html#%28form._%28%28quote._~23~25kernel%29._define-values%29%29"'

# But this works!
$ osascript -e 'tell application "chrome" to open location "file:///Applications/Racket_v6.7/doc/reference/define.html#%28form._%28%28quote._~23~25kernel%29._define-values%29%29"'

# Well, you also want the "activate" at the end:
$ osascript -e 'tell application "chrome" to open location "file:///Applications/Racket_v6.7/doc/reference/define.html#%28form._%28%28quote._~23~25kernel%29._define-values%29%29" activate'

Interestingly the generic open seems to work fine for http:. Also fine if a file: location is under your home directory instead of /Applications. Because security?1 Anyway, this is probably why Racket developers and power users haven’t noticed, if they’re building Racket (and its docs) from HEAD.

So I can use osascript if I know the default browser. How do I know that? Ugh. OK. This information seems to reside in Library/Preferences/ Read the JSON, find the correct entry, grab the browser part of, and hopefully we’re good.


Computer science mostly isn’t science.

Software engineering mostly isn’t engineering.

Sometimes success is simply scrolling.

  1. Security theater? I don’t see why it’s safe to load a page, but risky to scroll to an anchor in it? I also don’t see why /Applications is more risky than your home dir — much less some rando http: location?