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module Make
(I : MenhirLib.IncrementalEngine.EVERYTHING)
(User : sig
(* In order to submit artificial tokens to the parser, we need a function
that converts a terminal symbol to a token. Unfortunately, we cannot
(in general) auto-generate this code, because it requires making up
semantic values of arbitrary OCaml types. *)
val terminal2token: _ I.terminal -> I.token
end)
= struct
open MenhirLib.General
open I
open User
(* ------------------------------------------------------------------------ *)
(* Explanations. *)
type explanation = {
item: item;
past: (xsymbol * Lexing.position * Lexing.position) list
}
let item explanation =
explanation.item
let past explanation =
explanation.past
let future explanation =
let prod, index = explanation.item in
let rhs = rhs prod in
drop index rhs
let goal explanation =
let prod, _ = explanation.item in
lhs prod
(* ------------------------------------------------------------------------ *)
(* [items_current env] assumes that [env] is not an initial state (which
implies that the stack is non-empty). Under this assumption, it extracts
the automaton's current state, i.e., the LR(1) state found in the top
stack cell. It then goes through [items] so as to obtain the LR(0) items
associated with this state. *)
let items_current env : item list =
(* Get the current state. *)
match Lazy.force (stack env) with
| Nil ->
(* If we get here, then the stack is empty, which means the parser
is in an initial state. This should not happen. *)
invalid_arg "items_current" (* TEMPORARY it DOES happen! *)
| Cons (Element (current, _, _, _), _) ->
(* Extract the current state out of the top stack element, and
convert it to a set of LR(0) items. Returning a set of items
instead of an ['a lr1state] is convenient; returning [current]
would require wrapping it in an existential type. *)
items current
(* [is_shift_item t item] determines whether [item] justifies a shift
transition along the terminal symbol [t]. *)
let is_shift_item (t : _ terminal) (prod, index) : bool =
let rhs = rhs prod in
let length = List.length rhs in
assert (0 < index && index <= length);
(* We test that there is one symbol after the bullet and this symbol
is [t] or can generate a word that begins with [t]. (Note that we
don't need to worry about the case where this symbol is nullable
and [t] is generated by the following symbol. In that situation,
we would have to reduce before we can shift [t].) *)
index < length && xfirst (List.nth rhs index) t
let compare_explanations x1 x2 =
let c = compare_items x1.item x2.item in
(* TEMPORARY checking that if [c] is 0 then the positions are the same *)
assert (
c <> 0 || List.for_all2 (fun (_, start1, end1) (_, start2, end2) ->
start1.Lexing.pos_cnum = start2.Lexing.pos_cnum &&
end1.Lexing.pos_cnum = end2.Lexing.pos_cnum
) x1.past x2.past
);
c
(* [marry past stack] TEMPORARY comment *)
let rec marry past stack =
match past, stack with
| [], _ ->
[]
| symbol :: past, lazy (Cons (Element (s, _, startp, endp), stack)) ->
assert (compare_symbols symbol (X (incoming_symbol s)) = 0);
(symbol, startp, endp) :: marry past stack
| _ :: _, lazy Nil ->
assert false
(* [accumulate t env explanations] is called if the parser decides to shift
the test token [t]. The parameter [env] describes the parser configuration
before it shifts this token. (Some reductions have taken place.) We use the
shift items found in [env] to produce new explanations. *)
let accumulate (t : _ terminal) env explanations =
(* The parser is about to shift, which means it is willing to
consume the terminal symbol [t]. In the state before the
transition, look at the items that justify shifting [t].
We view these items as explanations: they explain what
we have read and what we expect to read. *)
let stack = stack env in
List.fold_left (fun explanations item ->
if is_shift_item t item then
let prod, index = item in
let rhs = rhs prod in
{
item = item;
past = List.rev (marry (List.rev (take index rhs)) stack)
} :: explanations
else
explanations
) explanations (items_current env)
(* TEMPORARY [env] may be an initial state!
violating [item_current]'s precondition *)
(* [investigate pos checkpoint] assumes that [checkpoint] is of the form
[InputNeeded _]. For every terminal symbol [t], it investigates
how the parser reacts when fed the symbol [t], and returns a list
of explanations. The position [pos] is where a syntax error was
detected; it is used when manufacturing dummy tokens. This is
important because the position of the dummy token may end up in
the explanations that we produce. *)
let investigate pos (checkpoint : _ checkpoint) : explanation list =
weed compare_explanations (
foreach_terminal_but_error (fun symbol explanations ->
match symbol with
| X (N _) -> assert false
| X (T t) ->
(* Build a dummy token for the terminal symbol [t]. *)
let token = (terminal2token t, pos, pos) in
(* Submit it to the parser. Accumulate explanations. *)
match shifts (offer checkpoint token) with
| None ->
explanations
| Some env ->
accumulate t env explanations
) []
)
(* We drive the parser in the usual way, but records the last [InputNeeded]
checkpoint. If a syntax error is detected, we go back to this checkpoint
and analyze it in order to produce a meaningful diagnostic. *)
exception Error of (Lexing.position * Lexing.position) * explanation list
let entry (start : 'a I.checkpoint) lexer =
let fail (inputneeded : 'a I.checkpoint) (checkpoint : 'a I.checkpoint) =
(* The parser signals a syntax error. Note the position of the
problematic token, which is useful. Then, go back to the
last [InputNeeded] checkpoint and investigate. *)
match checkpoint with
| HandlingError env ->
let (startp, _) as positions = positions env in
raise (Error (positions, investigate startp inputneeded))
| _ ->
assert false
in
I.loop_handle_undo Fun.id fail lexer start
(* TEMPORARY could also publish a list of the terminal symbols that
do not cause an error *)
end