Quick question

[Hannah Schroeter]

Hello!

On Wed, Nov 15, 2000 at 04:25:35PM +0100, Micha Pringle wrote:
> I'd like to make a simple infix expression evaluator.
> It should be able to support simple functions, such as *, +, -,/ as well as
> simpl recursion.
> That is, if I define
> G <- 19
> H <- G + 6
> evaluate G shoulr return 25
> Anyone know a good webpage for some info?

I don't know about a good web page. However nearly every book on
compiler design will handle examples like yours, usually using
parser and/or scanner generators.

You can do the same in Haskell, as there are parser generators (happy)
and scanner generators (alex) available.

For simple expressions as in your example you could evaluate the
commands immediately (if a command is either a variable assignment or
an expression), for example by having the parser for command *list*
return something like

type CommandListResult = [CommandResult]
type CommandResult = VariableState -> IO VariableState
type Variablestate = [(String, Value)]
type Value = Int -- or, if you have multiple, dynamic, value types,
  -- then it's data Value = ValInt Int | ...

However, if you want to define functions, do loop constructs, etc.,
you'll have a much easier way to build some syntax tree, perhaps
even transform it a bit, and then write an evaluator for the tree.

In interactive mode, you'll have to do it even a bit different.
Then, you must have a state containing
  function definitions
  variable contents
each command can define a function (altering the "function definitions"
part of the state), evaluate an expression (altering nothing except if
it calls a side-effecting function) or set a variable (altering the
"variable contents" part of the state). Probably, the function definitions
is an alist of name -> (list of formal parameters, some representation
of the function body)

Kind regards,

Hannah.