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Laser:-Static-Analysis-for-Ruby,-in-Ruby.md

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Synopsis

What truly makes Ruby special as a language is its focus on expressivity, flexibility, and dynamism. Yet these same properties - and their widespread use in the community - make even straightforward application code difficult to analyze statically in a meaningful way.

Laser seeks to change that. As a general-purpose analyzer using traditional compiler techniques, it statically discovers properties of real-world Ruby programs that no existing tool can. This talk is a whirlwind tour of what Laser can do, how it does it, and what it means for a typical Ruby programmer (who doesn't want to litter his or her code with type annotations). Among the questions it attempts to answer:

  • What code never runs?
  • What variables are unused?
  • What code runs, but has no meaningful effect?
  • What exceptions might a method raise?
  • Are blocks required, optional, or ignored by a method?
  • What variables are constant?
  • What methods get generated (or removed) by loops at load-time?
  • What types are being used, and where?
  • What gets added to a class by calling a method like acts_as_list?

Most importantly, Laser uses this information to find bugs and tell you about them, in addition to warning you about potential mistakes. It has a clear integration path with YARD and Redcar, as well as a possible future in optimization. On a broader scale, Laser exposes and builds upon the underlying strength and regularity of Ruby and modern Ruby techniques, without restricting Ruby's natural expressivity through static typing.

Laser

  • Runs Ruby code with Laser#Magic; not strictly static analysis
  • Ruby Object Model reimplemented in Laser
  • LaserMethod -- one instance per method in analyzed program
  • Cartesian Product Algorithm (by Agesen -- created for Smalltalk/Self)
    • Reasoning about types
    • Duplicating Cartesian graph on types
    • LaserMethod#return_type_for_types
  • Expected return types (warnings based on #to_s, #to_a, !, etc)
  • Unused methods based on the been_used flag. Again, it's running your code to figure this out.
  • Unused vars (in parser!). Can't tell dead code. :(
  • Other algo... missed its name. It's O(V) in running time.
  • Block usage. Seems crazy. Too much to write down.
    • Sends in a canary, tests to see if it's called like a block would be with yield, block_given?, iterator?, etc.
    • Checks recursively
  • Simulation
    • Interpreting Ruby in Ruby
    • Way slow.
  • CFG isn't conservative enough. Known to be wrong.

He's doing Java and Python at Google now. Not sure if Laser is going to do a ton more.