[epsilon-devel] Interpreter of an imperative language in epsilon

[Luca Saiu]

  As I promised I have written an interpreter for a very simple 
imperative language. The code should be clean and readable; you should
understand it before attempting to write a Logo system.
  I know that this may seem difficult if it's the first time you 
seriously attempt understanding an interpreter. Don't be afraid, it's 
not *so* difficult. It's easier than the interpreters in SICP, because 
there is no eval/apply loop here (and this is because the language is 
really, really trivial :-)).
  If you have any doubts don't hesitate to ask me.

  The main action is in main.epb; it contains some simple imperative 
programs that you can execute.
  The heart of the interpreter is in imperative.epi (for the abstract 
syntax) and especially in imperative.epb (for the semantics).

  Possible excercises for you:
* Understand how to write imperative programs in abstract syntax. You 
can use the sample programs in main.epb, and (especially) the concrete 
type definitions in imperative.epi.
* Add this imperative program (it computes the factorial) to main.epb as 
program5 and test it (don't use tabs for indenting :-))):
                input_integer n;
                i := 1;
                fact := 1;
                while i < n + 1 do {
                  fact := fact * i;
                  i := i + 1;
                }
                output fact;
* Read and understand all the code, with patience. This will take some time.
* Write more operators (string concatenation, less_or_equal, xor, and so 
on). Operators need not to be complex; the important matter is not what 
hey do, it's how easily you can add them to the language. Don't 
cut&paste here.
* Modify operators.epb, making the 'equal' operator polymorphic (it 
means that it should work not only for pairs of integers, but also for 
pairs of booleans and for pairs of strings). Hint: use is_value_XXXX.
* Add a construct "repeat <command> until <expression>" to the language.
* Add a construct "for <var> from <e1> to <e2> do <c>" (this is more 
difficult).

  Good night,

--
Luca Saiu, maintainer of GNU epsilon
http://www.gnu.org/software/epsilon

imperative.tar.gz
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