my own wishes for this little scheme
------------------------------------

* implement char=?, char<?, char>?, char<=?, and char>=?
  (char<? #\a #\b) => #t

* readline support (blah, use emacs it's good enough)

* eval code in any given binding (and, hence, expose the binding somehow)


tutorial exercises
------------------

* Add data types and parsers to support the full numeric tower of Scheme numeric types. Haskell has built-in
  types to represent many of these; check the Prelude. For the others, you can define compound types that
  represent eg. a Rational as a numerator and denominator, or a Complex as a real and imaginary part (each
  itself a Real number).

  http://www.schemers.org/Documents/Standards/R5RS/HTML/r5rs-Z-H-9.html#%_sec_6.2.1


* Add support for the backquote syntactic sugar: the Scheme standard details what it should expand into
  (quasiquote/unquote).

  http://www.schemers.org/Documents/Standards/R5RS/HTML/r5rs-Z-H-7.html#%_sec_4.2.6

* Add support for vectors. The Haskell representation is up to you: GHC does have an Array data type, but it
  can be difficult to use. Strictly speaking, a vector should have constant-time indexing and updating, but
  destructive update in a purely functional language is difficult. You may have a better idea how to do this
  after the section on set!, later in this tutorial.

  http://www.schemers.org/Documents/Standards/R5RS/HTML/r5rs-Z-H-9.html#%_sec_6.3.6
  http://www.haskell.org/ghc/docs/latest/html/libraries/base/Data-Array.html

* Instead of using the try combinator, left-factor the grammar so that the common subsequence is its own
  parser. You should end up with a parser that matches a string of expressions, and one that matches either
  nothing or a dot and a single expressions. Combining the return values of these into either a List or a
  DottedList is left as a (somewhat tricky) exercise for the reader: you may want to break it out into another
  helper function.


* Implement <strike>cond</strike> and case expressions.

* Add the rest of the string functions. You don't yet know enough to do string-set!; this is difficult to
  implement in Haskell, but you'll have enough information after the next 2 sections