[JohnBooty]

Think about this pseudocode; imagine it's The Hot New Dynamic Language Of The Week.

  print "Which do you like better: cars or trees?
  let a = AskUserForInput()
  var b
  if a=="cars" then
   b = new Car()
  elseif a=="trees" then
   b = new Tree()
  endif
  b.DriveOnExpressway()

Clearly, that's not going to work out too well if the user picks "trees" but it's tough to spot before run time.

While that's a contrived and simplistic example (that you probably could somewhat easily detect with tooling/analysis) think about a language like Javascript where there are no classes and you may be adding or modifying a bunch of an object's methods at runtime. At coding time, the tooling doesn't even know if foo has a .bar() function, much less which .bar() function, much less if the particular .bar() function that foo may have is being called with acceptable parameters.

edit: even my pseudocode is buggy ;-)

[JackC]

I was curious what PyCharm would handle that. Here's a screenshot: http://imgur.com/IvwG5

Basically it will complain if neither Car or Tree have a DriveOnExpressway method, but it's fine if one of them does -- the warning goes away if I uncomment the method. So there's ways to fall through the safety net, but lots of things it will catch.

(In Javascript, it's looser -- it only flags a potential problem if "DriveOnExpressway" isn't defined at all in the file.)

[jmmcd]

Ok, if I understand you right, your point is that a tool like a linter will fail to warn about this error. I'm not sure what current (say) Python linters do in situations like that.

But a tool like "go to definition" would work fine, wouldn't it? Maybe you would need a heuristic that guesses what are the possible types b could have, and detects that Tree doesn't have DriveOnExpressway, so it takes you straight to the definition in Car. Certainly, there are more complicated cases where guessing b's type is harder or impossible. I'm not denying that.