Contextual matching and transformations

[abhinav]

Patches currently apply at matching "levels" in the AST.

For example, consider the following patch.

@@
@@
-func foo(...) {
+func foo(...) error {
    ...
    if err != nil {
-       log.Fatal(err)
+       return err
    }
    ...
 }

In addition to the issue with replacing only the first instance of the matching statement (described in #10), this has the problem that the statement is only matched at the "top level" of the function. It will not affect the following versions, for example:

func foo() {
    for x := range y {
        err := bar(x)
        if err != nil {
            log.Fatal(err)
        }
    }
}

We need a means of specifying that given some previously matched context, we want to apply a "sub transformation" to the entire tree inside that context.

I've been referring to this concept as contextual matching but there may be a better term.

Borrowing from Coccinelle, one possible syntax here is <..., ...>. For example,

@@
@@
-func foo(...) {
+func foo(...) error {
    <...
      if err != nil {
-       log.Fatal(err)
+       return err
      }
    ...>
 }

Which I'm roughly translating to "given that matched context, apply this match/transformation to everything inside it." The "match all" nature of this syntax might also help address #10.