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9 ML Interface

The Twelf implementation defines a number of ML functions embedded in structures which can be called to load files, execute queries, and set environment parameters such as the verbosity level of the interaction. These functions and parameters are available in the Twelf structure. If you open the Twelf structure with

open Twelf

after compiling and loading Twelf, you do not have to type the `Twelf.' to the functions shown below.

Previous implementations of Elf offered a stand-alone command interpreter but this has not yet been ported. To exit Twelf and ML call Twelf.OS.exit ();.

9.1 Configurations

Groups of Twelf files are managed in configurations. A configuration is defined by a file, by convention called `sources.cfg', which resides in the same directory as the Twelf source files. The configuration file must contain at most one Twelf source file per line, and the files must be listed in dependency order. A configuration config can then be defined from the file by the ML declaration

val config = Twelf.Config.read "sources.cfg";

By convention, the filenames end in the extensions

for constant declarations and definitions or mixed files,
for files which contain query declarations,
for files which contain %theorem and %proof declarations.

File names may not contain whitespace. They are interpreted relative to the current working directory of ML, but resolved into absolute path names when the configuration file is read. To change the current working directory call

Twelf.OS.getDir ();               (* get working directory *)
Twelf.OS.chDir "directory"; (* change working directory *)

As an example, we show how the Mini-ML configuration is defined and loaded, assuming your current working directory is the root directory of Twelf.

val mini_ml = Twelf.Config.read "examples/mini-ml/sources.cfg";
Twelf.Config.load mini_ml;

Note that the identifier bound to the configuration (mini_ml in this example), must be a legal ML identifier, usually consisting only of alphanumeric characters and underscores. The call to Twelf.Config.load returns either Twelf.OK or Twelf.ABORT. It reads each file in turn, starting from an empty signature, printing the results of type reconstruction and search based on the value of the Twelf.chatter variable (see section 9.3 Environment Parameters). If another configuration or file has previously been read, all the declarations will first be deleted so that Twelf.Config.load always starts from the same state.

Loading a configuration will stop at the first error encountered, issue an appropriate message and return Twelf.ABORT. If there is an unexpected internal error (which indicates a bug in the Twelf implementation), it raises an uncaught exception instead and returns to the ML top-level.

To explore the behavior of programs interactively, you may call the Twelf top-level with

Twelf.top ();

which is explained in section 5.3 Interactive Queries.

9.2 Loading Files

Twelf also allows direct management of the signature by loading individual files. This is generally not recommended because successive declarations simply accumulate in the global signature which may lead to unexpected behavior. The relevant function calls are

Twelf.reset ();
Twelf.loadFile "file";

where Twelf.reset () resets the current global signature to be empty and Twelf.readFile "file" loads the given file whose name is interpreted relative to the current working directory.

Caution: Reading a file twice will not replace the declarations of the first pass by the second, but simply add them to the current signature. If names are reused, old declarations will be shadowed, but they are still in the global signature and might be used in the search for a solution to a query or in theorem proving, leading to unexpected behavior. When in doubt, use configurations (see section 9.1 Configurations) or call Twelf.reset ().

9.3 Environment Parameters

Various flags and parameters can be used to modify the behavior of Twelf and the messages it issues. They are given below with the assignment of the default value.

Twelf.chatter := 3;
Controls the detail of the information which is printed when signatures are read.
Just file names.
File names and number of query solutions.
Each declarations after type reconstruction.
Debug information.
More debug information.
Twelf.doubleCheck := false;
If true, each declaration is checked again for type correctness after type reconstruction. This is expensive and useful only for your peace of mind, since type checking is significantly simpler than type reconstruction.
Twelf.Print.implicit := false;
If true, implicit arguments (normally elided) are printed. Sometimes this is useful to track particularly baffling errors.
Twelf.Print.depth := NONE;
If SOME(d) then terms deeper than level d are printed as `%%'.
Twelf.Print.length := NONE;
If SOME(l) then argument lists longer than l are truncated with `...'.
Twelf.Print.indent := 3;
Controls the amount of indentation for printing nested terms.
Twelf.Print.width := 80;
The value used to decide when to break lines during printing of terms.
Twelf.Prover.strategy := Twelf.Prover.FRS;
Determines the strategy, where F=Filling, R=Recursion, and S=Splitting. Can also be Twelf.Prover.RFS.
Twelf.Prover.maxSplit := 2;
The maximal number of generations of a variable introduced by splitting. Setting is to 0 will prohibit proof by cases.
Twelf.Prover.maxRecurse := 10;
The maximal number of appeals to the induction hypothesis in any case during a proof.

9.4 Timing Statistics

Twelf has a few utilities to collect run-time statistics which are useful mainly for the developers. They are collected in the structure Timers. Timing information is cumulative in an ML session.

Twelf.Timers.show ();
Show the value of timers and reset them to zero.
Twelf.Timers.reset ();
Simply reset all timers to zero.
Twelf.Timers.check ();
Display the value of timers, but do not reset them.

Caution: Normally, the various times are exclusive, except that the runtime includes the garbage collection time which is shown separately. However, there is a problem the time for printing the answer substitution to a query is charged both to Printing and Solving.

9.5 Twelf Signature

For reference, here is the ML signature TWELF of the Twelf structure which defines most functions and flags relevant to loading and executing Twelf programs.

signature TWELF =
  structure Print :
    val implicit : bool ref           (* false, print implicit args *)
    val depth : int option ref        (* NONE, limit print depth *)
    val length : int option ref       (* NONE, limit argument length *)
    val indent : int ref              (* 3, indentation of subterms *)
    val width : int ref               (* 80, line width *)

  structure Timers :
    val show : unit -> unit           (* show and reset timers *)
    val reset : unit -> unit          (* reset timers *)
    val check : unit -> unit          (* display, but not no reset *)

  structure OS :
    val chDir : string -> unit        (* change working directory *)
    val getDir : unit -> string       (* get working directory *)
    val exit : unit -> unit           (* exit Twelf and ML *)

  structure Prover :
    datatype Strategy = RFS | FRS     (* F=Fill, R=Recurse, S=Split *)
    val strategy : Strategy ref       (* FRS, strategy used for %prove *)
    val maxSplit : int ref            (* 2, bound on splitting  *)
    val maxRecurse : int ref          (* 10, bound on recursion *)

  val chatter : int ref               (* 3, chatter level *)
  val doubleCheck : bool ref          (* false, check after reconstruction *)

  datatype Status = OK | ABORT        (* return status *)

  val reset : unit -> unit            (* reset global signature *)
  val loadFile : string -> Status     (* load file *)
  val readDecl : unit -> Status       (* read declaration interactively *)
  val decl : string -> Status         (* print declaration of constant *)

  val top : unit -> unit              (* top-level for interactive queries *)

  structure Config :
    type config                       (* configuration *)
    val read : string -> config       (* read config file *)
    val load : config -> Status       (* reset and load configuration *)
    val define : string list -> config (* explicitly define configuration *)

  val version : string                (* Twelf version *)
end;  (* signature TWELF *)

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