String substitutions in ruby and coding style

Question

I wrote a short ruby script to generate various screenshots, and event though I am not completely new to ruby I rarely use it.

Full script (41 LOC): https://gist.github.com/1229115

The string substitutions irritate me. They don't look quite right - much too ugly for a beautiful and concise language like ruby. Am I overdoing it? What alternatives are there?

Here is the lines I am concerned about (for context, please take a look at the gist):

# assemble file name
fname = "#{directory}/#{prefix}-#{timestamp}-#{commit}-#{size[:width]}x#{size[:height]}.png"
# create command line stuff
call = "#{macruby_exe} #{snapper} \
 --width #{size[:width]} --height #{size[:height]} #{url} #{fname}"
# now call it
system call
# just to show the progress (and the file sizes)
puts "#{fname} (#{File.size?(fname) / 1024}kB)"

---

Just to illustrate the script, the output usually looks like:

screenies/screenshot-1316535022-0b5c7967887481deb17bcd9039cd3fe0ac4540d4-640x960.png (95kB)
screenies/screenshot-1316535022-0b5c7967887481deb17bcd9039cd3fe0ac4540d4-800x480.png (62kB)
screenies/screenshot-1316535022-0b5c7967887481deb17bcd9039cd3fe0ac4540d4-800x600.png (78kB)
 ^ ^ ^ ^ ^
 | | | | ________|______
 | | | | | |
directory prefix timestamp commit size[:width] x size[:height]

Answer 1

There are other ways you could write this but I'm not sure if you'd find any of them cleaner. One option is plain old sprintf. For example:

# assemble file name
fname = sprintf("%s/%s-%i-%s-%ix%i.png", directory, prefix, timestamp,
 commit, size[:width], size[:height])

You could shorten it a little by giving size its own to_s method:

class << size; def to_s; "#{self[:width]}x#{self[:height]}" end end
fname = sprintf("%s/%s-%i-%s-%s.png", directory, prefix, timestamp,
 commit, size)

...and perhaps create other convenience methods along the same lines. But still, very small gains.

Another option would be to make your own super-small template engine, something like this:

def tiny_subst template, bndg
 param_exp = /:(\w+)[^\w:]+/i
 tmpl_params = template.scan(param_exp).flatten
 proc {
 out_str = template.clone
 tmpl_params.reduce(out_str) do |str, p|
 str.gsub ":#{p}", eval(p, bndg).to_s unless p.empty?
 end
 }
end
tmpl = ':directory/:prefix-:timestamp-:commit-:size.png'
subber = tiny_subst(tmpl, binding)
directory = 'screenies'
prefix = 'screenshot'
timestamp = Time.now.to_i
commit = '0b5c7967887481deb17bcd9039cd3fe0ac4540d4'
size = '640x960'
subber.yield # => screenies/screenshot-1316535022-0b5c7967887481deb17bcd9039cd3fe0ac4540d4-640x960.png
commit = 'foobarbazquux'
size = '9000x9000'
subber.yield # => screenies/screenshot-1316535022-foobarbazquux-9000x9000.png

So, that's kinda nice, but is it really worth it? Not for me, but I guess if you really hate "#{this}" it might be.

• miku: Thank you - an inspiring answer. I don't really hate #{this} but the mini-template seems nicer, I'll try it.
• Jordan: Well it's not really intended for use but if you find it useful I actually put it up on GitHub last night: github.com/jrunning/StupidLittleTemplate There are probably security implications to eval-ing variables in the local scope, so please don't use it in a production app, or at the very least sanitize user data until it sparkles before passing it in.
• miku: Great, I'll take a look.

Answer 2

Per your request (and drawing from your gist), more concise and readable is:

screensizes = [
# width height
 [ 640, 960 ], # iPhone 4
 [ 800, 480 ], # wvga
 [ 800, 600 ], # old school
 [ 1024, 600 ], # netbook, WSVGA
 [ 1024, 786 ], # old school, iPad
 [ 1200, 800 ], # macbook
 [ 1366, 786 ], # netbook
 [ 1440, 900 ], # macbook pro
 [ 1600, 768 ], # strange
 [ 1680, 1050 ], # hires macbok
]
screensizes.each do |w,h|
# Assemble file name:
 f = "#{directory}/#{prefix}-#{timestamp}-#{commit}-#{w}x#{h}.png"
# Create command line stuff:
 s = "#{macruby_exe} #{snapper} --width #{w} --height #{h} #{url} #{f}"
# Now call it:
 system s
# Show progress and file sizes:
 puts "#{f} (#{File.size?(f) / 1024}kB)"
end