Re: [Caml-list] Error Reporting

["David McClain" <barabh@qwest.net>]

Um, perhaps I misled everybody by claiming the compiler is reporting the
errors....

What I meant to say was that my compiler compiles a tree of closures passing
runtime control from one closure to the next with a CPS protocol. The errors
I am interested in reporting happen later at runtime when these trees are
executed.

The errors come as a result of invalid data types and possible user
interruptions of looping code. This compiler is NOT strongly typed, but
rather, behaves more like Lisp in that regard. It allows for dynamic typing
and sometimes this leads to program failure when inappropriate operations
are performed on data items.

So perhaps we can now get back on the same sheet of music. I see where you
thought I already had the information needed if, by error reporting, I had
meant that the compiler reports the errors. My compiler does quite well at
reporting program structural errors. The runtime errors of later execution
are the errors on which I am interested in building traceback information.

Thanks,

- DM

----- Original Message -----
From: "Krishnaswami, Neel" <neelk@cswcasa.com>
To: <caml-list@inria.fr>
Sent: Thursday, September 27, 2001 6:32 AM
Subject: Re: [Caml-list] Error Reporting


> David McClain [mailto:barabh@qwest.net] wrote:
> >
> > I am looking for some ideas on handling program error reporting for
> > a compiler that is built largely on CPS. Since there are no stack
> > frames to crawl for traceback, I have a first cut based on keeping a
> > finite length queue of last visited closures. But this becomes
> > problematic when there are circularities in the code, i.e.,
> > recursion. Also, I think I have way too much error tracking going on
> > at each incremental execution step and so runtime performance is cut
> > back compared to what it could be.
>
> Since you're using CPS, it seems like you have most of the infrastructure
> you need already. If you track source locations from parsing down to code
> generation, you can place a source location into each frame as you
allocate
> it. Then, when you have an error, you can simply walk back down the
> contination, emitting the source location of each frame. Since the
> continuation represents the rest of the computation, you automatically get
> the exact execution context of the erroneous statement.
>
> The overhead for this is one word per frame, plus initializing it
> at each continuation allocation. This is probably acceptable unless
> you are writing a really high-performance compiler. Also, tail-call
> optimization will make the traceback only approximate, but this is
> IME not a big deal in practice.
>
> Below is a small CPS-style lambda calculus interpreter that uses this
> idea to generate tracebacks of its evaluation. It's a treecode
> interpreter, but the ideas ought to carry over directly to bytecode too.
>
> -*-*-*-
>
> type var = string
> type pos = int
>
> (* Expressions carry the source location with them *)
>
> type expr =
>   | Lit of pos * int
>   | Var of pos * var
>   | Fun of pos * var * expr
>   | App of pos * expr * expr
>
> (*
>    Continuation frames have the position as an extra argument, so that
>    the continuation as a whole contains the backtrace info. Primitive
>    functions take the continuation as an extra argument so that they
>    can generate a backtrace on an error.
> *)
>
> type denot =
>   | Int of int
>   | Closure of env * var * expr
>   | Primop of (denot -> cont -> denot)
> and frame =
>   | KFun of pos * env * expr
>   | KArg of pos * env * denot
> and env = (var * denot) list
> and cont = frame list
>
> (*
>    Here are the guts of the machinery to generate a backtrace. We
>    need to extract a position from a frame and then just map over
>    the whole continuation to get the backtrace.
> *)
>
> exception Eval_error of string * pos list
>
> let framepos frame =
>   match frame with
>   | KFun(pos, _, _) -> pos
>   | KArg(pos, _, _) -> pos
>
> let backtrace k = List.map framepos k
>
> (*
>   We augment variable lookup so that failed lookups generate backtraces.
>   If you check statically for unbound variables this is unneccesary.
> *)
>
> let bind v d r = (v,d) :: r
>
> let lookup v r k =
>   try
>     List.assoc v r
>   with
>     Not_found -> raise (Eval_error("Unbound variable " ^ v, backtrace k))
>
>
> (*
>    In the interpreter proper, the only place where a backtrace is
>    generated is when an argument is applied to a non-function. If an
>    if-statement is added, then that should be too. Likewise, if
>    exceptions are added then an uncaught exception should generate
>    one also.
> *)
>
> let rec eval e r k =
>   match e with
>   | Lit(pos, n) ->
>       next (Int n) k
>   | Var(pos, v) ->
>       next (lookup v r k) k
>   | Fun(pos, formal, body) ->
>       next (Closure(r, formal, body)) k
>   | App(pos, f, arg) ->
>       eval f r (KFun(pos, r, arg) :: k)
> and next d k =
>   match k with
>   | [] -> d
>   | KFun(pos, r, arg) :: k' ->
>       eval arg r k'
>   | KArg(pos, r, func) :: k' ->
>       (match func with
>       | Int _ ->
>           raise (Eval_error("Not a function!", backtrace k))
>       | Closure(r, formal, body) ->
>           eval body (bind formal d r) k'
>       | Primop f ->
>           next (f d k) k')
> --
> Neel Krishnaswami
> neelk@cswcasa.com
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