Commit d0dfcde5 authored by POTTIER Francois's avatar POTTIER Francois
Browse files

Added [FixSolver], which solves a system of inequations in a join-semi-lattice.

Tested it by using to compute the FOLLOW sets.
parent 4eeb62da
module Make
(P : sig
include Fix.PROPERTY
val union: property -> property -> property
= struct
type variable =
type property =
(* A constraint is represented as a mapping of each variable to an
expression, which represents its lower bound. We could represent
an expression as a list of constants and variables; we can also
represent it as a binary tree, as follows. *)
type expression =
| EBottom
| ECon of property
| EVar of variable
| EJoin of expression * expression
type constraint_ =
expression M.t
(* Looking up a variable's lower bound. *)
let consult (m : constraint_) (x : variable) : expression =
M.find x m
with Not_found ->
(* Evaluation of an expression in an environment. *)
let rec evaluate get e =
match e with
| EBottom ->
| ECon p ->
| EVar x ->
get x
| EJoin (e1, e2) ->
P.union (evaluate get e1) (evaluate get e2)
(* Solving a constraint. *)
let solve (m : constraint_) : variable -> property =
let module F = Fix.Make(M)(P) in
F.lfp (fun x get ->
evaluate get (consult m x)
(* The imperative interface. *)
let create () =
let m = M.create() in
let record_ConVar p y =
M.add y (EJoin (ECon p, consult m y)) m
and record_VarVar x y =
M.add y (EJoin (EVar x, consult m y)) m
fun () -> solve m
module Make
(P : sig
include Fix.PROPERTY
val union: property -> property -> property
: sig
(* Variables and constraints. A constraint is an inequality between
a constant or a variable, on the left-hand side, and a variable,
on the right-hand side. *)
type variable =
type property =
(* An imperative interface, where we create a new constraint system,
and are given three functions to add constraints and (once we are
done adding) to solve the system. *)
val create: unit ->
(property -> variable -> unit) *
(variable -> variable -> unit) *
(unit -> (variable -> property))
......@@ -951,63 +951,6 @@ end = struct
(* ------------------------------------------------------------------------ *)
(* The computation of FOLLOW sets does not follow the above model. Instead, we
need to explicitly compute a system of equations over sets of terminal
symbols (in a first pass), then solve the constraints (in a second
pass). *)
(* The computation of the symbolic FOLLOW sets follows the same pattern, but
produces sets of symbols, instead of sets of terminals. For this reason,
we parameterize this little equation solver over a module [P], which we
later instantiate with [TerminalSet] and [SymbolSet]. *)
module Solve (P : sig
include Fix.PROPERTY
val union: property -> property -> property
end) = struct
(* An equation's right-hand side is a set expression. *)
type expr =
| EVar of Nonterminal.t
| EConstant of
| EUnion of expr * expr
(* A system of equations is represented as an array, which maps nonterminal
symbols to expressions. *)
type equations =
expr array
(* This solver computes the least solution of a set of equations. *)
let solve (eqs : equations) : Nonterminal.t -> =
let rec expr e get =
match e with
| EVar nt ->
get nt
| EConstant c ->
| EUnion (e1, e2) ->
P.union (expr e1 get) (expr e2 get)
let nonterminal nt get =
expr eqs.(nt) get
let module F =
F.lfp nonterminal
(* ------------------------------------------------------------------------ *)
(* Compute which nonterminals are nonempty, that is, recognize a
nonempty language. Also, compute which nonterminals are
......@@ -1125,8 +1068,9 @@ let () =
on demand. *)
(* The computation of the symbolic FOLLOW sets follows exactly the same
pattern. We share code and parameterize this computation over a module [P],
just like the little equation solver above. *)
pattern as that of the traditional FOLLOW sets. We share code and
parameterize this computation over a module [P]. The type []
intuitively represents a set of symbols. *)
module FOLLOW (P : sig
include Fix.PROPERTY
......@@ -1135,21 +1079,23 @@ module FOLLOW (P : sig
val first: Production.index -> int -> property
end) = struct
module S = Solve(P)
open S
module S =
(* First pass. Build a system of equations. *)
(* Build a system of constraints. *)
let follow : equations =
Array.make Nonterminal.n (EConstant P.bottom)
let record_ConVar, record_VarVar, solve =
(* Iterate over all start symbols. *)
let () =
let sharp = EConstant (P.terminal in
let sharp = P.terminal in
for nt = 0 to Nonterminal.start - 1 do
assert (Nonterminal.is_start nt);
(* Add # to FOLLOW(nt). *)
follow.(nt) <- EUnion (sharp, follow.(nt))
record_ConVar sharp nt
(* We need to do this explicitly because our start productions are
of the form S' -> S, not S' -> S #, so # will not automatically
......@@ -1168,18 +1114,18 @@ end) = struct
and first = P.first prod (i+1) in
(* The FIRST set of the remainder of the right-hand side
contributes to the FOLLOW set of [nt2]. *)
follow.(nt2) <- EUnion (EConstant first, follow.(nt2));
record_ConVar first nt2;
(* If the remainder of the right-hand side is nullable,
FOLLOW(nt1) contributes to FOLLOW(nt2). *)
if nullable then
follow.(nt2) <- EUnion (EVar nt1, follow.(nt2))
record_VarVar nt1 nt2
) rhs
) Production.table
(* Second pass. Solve the equations (on demand). *)
let follow : Nonterminal.t -> =
solve follow
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