using LC as an emulator

[Mark Waddingham]

On 2017-11-07 17:42, Mike Kerner via use-livecode wrote:
> No, the ERP is written in HP BASIC, so I would want to emulate the
> language, given the breadth and number of programs.  I was thinking 
> about
> writing wrappers for the various commands and functions of the 
> language,
> but it won't be easy, no matter what I do.

In this case 'ease' is probably related to the breadth of the language 
and how complex individual commands/functions are - if you have a good 
architecture for the emulator, most of the work will be in recreating 
that functionality, rather than the parsing/execution mechanism.

> There are also binary resources that I would have to have recreated 
> such as
> forms and dialogs, and I would have to take the db schemas and convert 
> them
> to a modern database, which seems like the least work of this entire 
> silly
> idea.

 From what you've said it doesn't sound like a silly idea at all.

> Would it be less work to rewrite it?  I doubt it.  Would it be less 
> effort
> to cough up for an off-the-self ERP package?  Possibly, but the one 
> piece
> that is extremely valuable is the payroll piece, because it is so 
> expensive
> to hire a service or to pay for someone else's payroll software.  For
> example, look at the time and attendance piece.  I was able to write a
> timeclock app that runs on a tablet for a tiny fraction of what it 
> would
> cost to replace the existing timeclocks.  Timekeeping software is also
> crazy expensive, and payroll is akin to highway robbery.

If it is a system you've been maintaining for 30 years, then I suspect 
you are correct - it won't be less work to rewrite as you'll spend a lot 
of time making things work as they did before... After all, I suspect 
that after 30 years working on it even you can't remember where all the 
bodies are buried!

(I certainly find zombies popping up now and again to bite me in the 
LiveCode engine from things I've done in the past!)

> Would it be easier to try to build an emulator in Xojo?  I've thought 
> about
> it, but before I add yet another development tool to the mix, here, I
> thought I'd chase this idea, first.

Well I can certainly say that LiveCode is perfectly capable, and indeed 
very good for writing compilers/interpreters because of the way its 
arrays work (copy-on-write in particular), and the ability to pass 
references to array elements to functions. (I've writing a few such 
things in the last 12 months - the SVG compiler being the most recent 
example).

I'd generally recommend using a LiveCode array to represent the source 
structure (after parsing - which will be a tree), and then write a 
recursive evaluator for it, threading through the node which is being 
executed and a mutable context array which contains all the current 
execution state:

   on langExecute @xContext, pNode
     - dispatch to do what you need to do for node, using state from 
xContext
   end langExecute

If you can figure out how to map the source language to (essentially) a 
sequence of command invocations, then you can write the each piece of 
functionality provided by the language (i.e. the bits other than 
variable manipulations and control transfer) as a single handler which 
takes the xContext parameter.

Okay, so all the above is a bit vague - but it is just to give an idea 
of how I've found the best way to structure such things in LiveCode.

Warmest Regards,

P.S. Do you have the online links for the reference manuals? I'm 
intrigued...

-- 
Mark Waddingham ~ mark at livecode.com ~ http://www.livecode.com/
LiveCode: Everyone can create apps