IncrementalEngine.ml 7.64 KB
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open General

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(* This signature describes the incremental LR engine. *)

(* In this mode, the user controls the lexer, and the parser suspends
   itself when it needs to read a new token. *)

module type INCREMENTAL_ENGINE = sig

  type token

  (* The type ['a result] represents an intermediate or final result of the
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     parser. An intermediate result is a suspension: it records the parser's
     current state, and allows parsing to be resumed. The parameter ['a] is
     the type of the semantic value that will eventually be produced if the
     parser succeeds. *)

  (* [Accepted] and [Rejected] are final results. [Accepted] carries a
     semantic value. *)

  (* [InputNeeded] is an intermediate result. It means that the parser wishes
     to read one token before continuing. *)

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  (* [Shifting] is an intermediate result. It means that the parser is taking
     a shift transition. It exposes the state of the parser before and after
     the transition. The Boolean parameter tells whether the parser intends to
     request a new token after this transition. (It always does, except when
     it is about to accept.) *)

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  (* [AboutToReduce] is an intermediate result. It means that the parser is
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     about to perform a reduction step. It exposes the parser's current
     state as well as the production that is about to be reduced. *)
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  (* [HandlingError] is an intermediate result. It means that the parser has
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     detected an error and is currently handling it, in several steps. *)
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  type env
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  type production

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  type 'a result = private
    | InputNeeded of env
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    | Shifting of env * env * bool
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    | AboutToReduce of env * production
    | HandlingError of env
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    | Accepted of 'a
    | Rejected

  (* [offer] allows the user to resume the parser after it has suspended
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     itself with a result of the form [InputNeeded env]. [offer] expects the
     old result as well as a new token and produces a new result. It does not
     raise any exception. *)
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  val offer:
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    'a result ->
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    token * Lexing.position * Lexing.position ->
    'a result

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  (* [resume] allows the user to resume the parser after it has suspended
     itself with a result of the form [AboutToReduce (env, prod)] or
     [HandlingError env]. [resume] expects the old result and produces a new
     result. It does not raise any exception. *)
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  val resume:
    'a result ->
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    'a result

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  (* The abstract type ['a lr1state] describes the non-initial states of the
     LR(1) automaton. The index ['a] represents the type of the semantic value
     associated with this state's incoming symbol. *)

  type 'a lr1state

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  (* An element is a pair of a non-initial state [s] and a semantic value [v]
     associated with the incoming symbol of this state. The idea is, the value
     [v] was pushed onto the stack just before the state [s] was entered. Thus,
     for some type ['a], the type [s] has type ['a lr1state] and the value [v]
     has type ['a]. In other words, the type [element] is an existential type. *)

  type element =
    | Element: 'a lr1state * 'a * Lexing.position * Lexing.position -> element

  (* The parser's stack is (or, more precisely, can be viewed as) a stream of
     elements. *)

  type stack =
    element stream

  (* The parser's stack, a stream of elements, can be examined. This stream is
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     empty if the parser is in an initial state; otherwise, it is non-empty.
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     The LR(1) automaton's current state is the one found in the top element
     of the stack. *)
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  val stack: env -> stack
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end
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(* This signature is a fragment of the inspection API that is made available
   to the user when [--inspection] is used. This fragment contains type
   definitions for symbols. *)

module type SYMBOLS = sig

  (* The type ['a terminal] represents a terminal symbol. The type ['a
     nonterminal] represents a nonterminal symbol. In both cases, the index
     ['a] represents the type of the semantic values associated with this
     symbol. The concrete definitions of these types are generated. *)

  type 'a terminal
  type 'a nonterminal

  (* The type ['a symbol] represents a terminal or nonterminal symbol. It is
     the disjoint union of the types ['a terminal] and ['a nonterminal]. *)

  type 'a symbol =
    | T : 'a terminal -> 'a symbol
    | N : 'a nonterminal -> 'a symbol

  (* The type [xsymbol] is an existentially quantified version of the type
     ['a symbol]. This type is useful in situations where the index ['a]
     is not statically known. *)

  type xsymbol = 
    | X : 'a symbol -> xsymbol

end

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(* This signature describes the inspection API that is made available to the
   user when [--inspection] is used. *)
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module type INSPECTION = sig

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  (* The types of symbols are described above. *)

  include SYMBOLS

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  (* Ordering functions. *)

  val compare_terminals: _ terminal -> _ terminal -> int
  val compare_nonterminals: _ nonterminal -> _ nonterminal -> int
  val compare_symbols: xsymbol -> xsymbol -> int
  val compare_words: xsymbol list -> xsymbol list -> int

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  (* The type ['a lr1state] is meant to be the same as in [INCREMENTAL_ENGINE]. *)

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  type 'a lr1state
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  (* The type [production] is meant to be the same as in [INCREMENTAL_ENGINE].
     It represents a production of the grammar. A production can be examined
     via the functions [lhs] and [rhs] below. *)

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  type production

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  (* An LR(0) item is a pair of a production [prod] and a valid index [i] into
     this production. That is, if the length of [rhs prod] is [n], then [i] is
     comprised between 0 and [n], inclusive. *)

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  type item =
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      production * int

  (* [incoming_symbol s] is the incoming symbol of the state [s], that is,
     the symbol that the parser must recognize before (has recognized when)
     it enters the state [s]. This function gives access to the semantic
     value [v] stored in a stack element [Element (s, v, _, _)]. Indeed,
     by case analysis on the symbol [incoming_symbol s], one discovers the
     type ['a] of the value [v]. *)
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  val incoming_symbol: 'a lr1state -> 'a symbol
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  (* [lhs prod] is the left-hand side of the production [prod]. This is
     always a non-terminal symbol. *)

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  val lhs: production -> xsymbol

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  (* [rhs prod] is the right-hand side of the production [prod]. This is
     a (possibly empty) sequence of (terminal or nonterminal) symbols. *)

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  val rhs: production -> xsymbol list

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  (* [items s] is the set of the LR(0) items in the LR(0) core of the LR(1)
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     state [s]. This set is not epsilon-closed. This set is presented as a
     list, in an arbitrary order. *)
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  val items: _ lr1state -> item list
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  (* [nullable nt] tells whether the non-terminal symbol [nt] is nullable.
     That is, it is true if and only if this symbol produces the empty
     word [epsilon]. *)

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  val nullable: _ nonterminal -> bool
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  (* [first nt t] tells whether the FIRST set of the nonterminal symbol [nt]
     contains the terminal symbol [t]. That is, it is true if and only if
     [nt] produces a word that begins with [t]. *)

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  val first: _ nonterminal -> _ terminal -> bool

  (* [xfirst] is analogous to [first], but expects a first argument of type
     [xsymbol] instead of [_ terminal]. *)

  val xfirst: xsymbol -> _ terminal -> bool
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  (* [foreach_terminal] enumerates the terminal symbols, including [error].
     [foreach_terminal_but_error] enumerates the terminal symbols, excluding
     [error]. *)

  val foreach_terminal:           (xsymbol -> 'a -> 'a) -> 'a -> 'a
  val foreach_terminal_but_error: (xsymbol -> 'a -> 'a) -> 'a -> 'a

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end

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(* This signature combines the incremental API and the inspection API. *)

module type EVERYTHING = sig

  include INCREMENTAL_ENGINE

  include INSPECTION
    with type 'a lr1state := 'a lr1state
    with type production := production

end