Question

Is it possible to mark a block in Vim based on the indentation already in place? Similarly to v{ .

It would be extremely useful for programming languages with whitespace-sensitive syntax (like Haskell and Python).

For example mark everything between the first let and return in this function:

checkArg (com:arg) s d ns 
  | com == "add-source " = do
      let s' = v ++ s
      lift $ saveLinks s'
      return (s', d)
  | com == "remove-source" = do
      let s' = filter (not . hasWord str) s
      lift $ saveLinks s'
      return (s', d)

http://en.wikipedia.org/wiki/Off-side_rule

Answer 1

I use the indent object plugin:

This plugin defines a new text object, based on indentation levels.
This is very useful in languages such as Python, in which the syntax
defines scope in terms of indentation. Using the objects defined in
this plugin, an entire if structure can be quickly selected, for
example.

With this, you can select, delete, change, etc. blocks using the standard Vim text object commands, using “i” and “a” to refer to the block that you are in: “vii”, “dii”, etc.
It it language-agnostic, though is especially useful/relevant in whitespace-structured languages such as Python.