I've been watching Philip Wadler videos over and over again to get a handle
on monads. But a question has arisen that I can't find an easy answer to.
Is there in general an easy way to combine multiple monads together?
Say I want both State *and* Output and I have a monad that produces output,
can I add State transformations to it or do I have to rewrite the whole program?
The specific case is I'm working with hand-written parser combinators
in PostScript that are modeled after the excellent paper by Graham and Hutton.
I'm trying to use these parsers to make a scanner for PostScript syntax.
At the top level I have handlers attached which produce an array of
[ original-string ps-object ] pairs. eg.
/ps-token
//rad-integer { cvi } handle
//real { cvr } handle plus
//integer { cvi } handle plus
//name { cvn cvx } handle plus
(/) char //name seq { rest cvn } handle plus
(/) char (/) char seq //name seq
{ rest rest cvn
dup {load}stopped not{ exch pop }if } handle plus
//ps-string { 1 1 index length 2 sub getinterval
} handle plus
//delimiter { cvn cvx } handle plus
def
But a complication arises when dealing with escapes in strings.
I can't eliminate the backslash character at the point where it's
found because the outer level code needs the original string
intact to advance its pointer in the input string.
So my options seem to be: add an extra pass to clean up string
data later, or augment the whole system to carry the input
position State around.
If there's some theoretical way to do it, I'd love to augment
my parser monad with State without rewriting the whole program.
Any thoughts or pointers appreciated.
https://github.com/luser-dr00g/pcomb/blob/master/ps/pc9token.ps
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