One random little feature of GNAT that comes in handy for me is its
habit of, when I misspell an identifier, giving me a possible correction
in its compile error message.  Spending some time with the 3.10.0
sources, I have created a "second draft" patch creating this
functionality in my favored language.

Example:
========

# /home/thelema/Projects/ocaml-custom/bin/ocamlc -o coml -I +lablgtk2
lablgtk.cma gtkInit.cmo coml.ml
File "coml.ml", line 61, characters 16-25:
Unbound value is_arcive, possible misspelling of is_archive

Impacts:
========

Efficiency in the case of finding a mistake should be quite good,
although this shouldn't matter too much since the compiler quits pretty
early in compilation when it finds an unbound identifier.

In the case of no unbound identifiers, the cost is an extra try/with
block around the standard lookup.  I haven't made any benchmarks, though.

I expect this code to have little long term maintenance issues - the
major source of code changes was adding a "* string list" to a number of
exceptions to carry the list of possible correct spellings to the point
they get output by the compiler.  These exceptions are still usable as
before with an empty list in this spot.

It's possible the code has created opportunities for uncaught exceptions
in the compiler as I only checked for instances of "Not_found" in a few
files -- those which dealt with the Unbound_* exceptions.  Someone who
knows the internals better might find places the "Found_nearly"
exception that carries possible corrections might escape into.


Dedicated to:
Yaron Minsky and the team at Jane Street

E.