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 ();.
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
%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.
Twelf.make "examples/mini-ml/sources.cfg";
The call to Twelf.make 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 11.3 Environment Parameters). If another configuration or file has previously been
read, all the declarations will first be deleted so that
Twelf.make always starts from the same state.
To load a configuration, use Twelf.Config.load config.
This is will first reset the state and then load config.
In order to avoid resetting the state, use
Twelf.Config.append config instead.
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.
The default suffix for configuration files is `cfg'; it can be changed with
Twelf.Config.suffix := "suffix";
although this may confuse the Twelf server.
Twelf configurations can also be defined explicitly from a list of file names with
Twelf.Config.define ["file1, ..., filen"];
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 11.1 Configurations) or
call Twelf.reset ().
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;
0
1
2
3
4
5
6
Twelf.doubleCheck := false;
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.unsafe := false;
true it will allow the %assert declaration to assert
theorems without proof.
Twelf.Print.implicit := false;
true, implicit arguments (normally elided) are printed.
Sometimes this is useful to track particularly baffling errors.
Twelf.Print.depth := NONE;
SOME(d) then terms deeper than level d are printed
as `%%'.
Twelf.Print.length := NONE;
SOME(l) then argument lists longer than l are truncated
with `...'.
Twelf.Print.indent := 3;
Twelf.Print.width := 80;
Twelf.Trace.detail := 1;
Twelf.Compile.optimize := true;
false, unification can be traced in more detail.
Twelf.Prover.strategy := Twelf.Prover.FRS;
F=Filling, R=Recursion, and
S=Splitting. Can also be Twelf.Prover.RFS.
Twelf.Prover.maxSplit := 2;
0 will prohibit proof by cases.
Twelf.Prover.maxRecurse := 10;
Twelf.Table.strategy := Twelf.Table.Variant;
Twelf.Table.Variant or Twelf.Table.Subsumption.
Twelf.Table.strengthen := false;
Twelf provides two ways to print the current global signature.
Twelf.Print.sgn (); Twelf.Print.prog ();
The first prints the signature, using only forward arrows ->, the
second will print the signature interpreted as a logic programming using
backward arrows <-. Depending on your goals, one or the other
might be easier to use.
Output can also be generated in TeX format. The necessary library files can be found in the `tex/' subdirectory of the distribution. To print the current signature using TeX format, use
Twelf.Print.TeX.sgn (); Twelf.Print.TeX.prog ();
with the same interpretation as the plain text printing commands above.
Twelf no incorporates some rudimentary tracing facilities for the logic
programming interpreter of signatures. This is best used within the
Emacs server, but it is also available within the ML Interface. It is
not available with the tabled logic programming interpreter. Also, when
optimizations are enabled (Twelf.Compile.optimize is
true), unification can not be traced.
A tracing specification may be associated with constants in a signature.
Twelf.Trace.None
Twelf.Trace.Some ["c1",...,"cn"]
Twelf.Trace.All
One can either suspend the execution when a specified clause or predicate is invoked, or simply trace goals
Twelf.Trace.trace spec
Twelf.Trace.break spec
When a breakpoint is set, execution will halt and ask for an action from the user. This consists of a (possible empty) line of input followed by RET. Current, the following actions are available.
<newline> - continue --- execute with current settings n - next --- take a single step r - run --- remove all breakpoints and continue s - skip --- skip until current subgoals succeeds, is retried, or fails s n - skip to n --- skip until goal (n) is considered t - trace --- trace all events u - untrace --- trace no events d n - detail --- set trace detail to n (0, 1, or 2) h - hypotheses --- show current hypotheses g - goal --- show current goal i - instantiation --- show instantiation of variables in current goal v X1 ... Xn - variables --- show instantiation of X1 ... Xn ? for help
The detail of the trace information can be set with the
variable Trace.detail := n; to one of
0
1
2
Note that if Twelf.Compile.optimize is true, then details
of unification cannot be shown. It is possible to examine and reset the
state of the currently traced predicates with
Twelf.Trace.show ();
Twelf.Trace.reset ();
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 ();
Twelf.Timers.reset ();
Twelf.Timers.check ();
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.
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 =
sig
structure Print :
sig
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 *)
val sgn : unit -> unit (* print signature *)
val prog : unit -> unit (* print signature as program *)
val subord : unit -> unit (* print subordination relation *)
val domains : unit -> unit (* list constraint domains *)
structure TeX : (* print in TeX format *)
sig
val sgn : unit -> unit (* print signature *)
val prog : unit -> unit (* print signature as program *)
end
end
structure Trace :
sig
datatype 'a Spec = (* trace and breakpoint spec *)
None (* no tracing, default *)
| Some of 'a list (* list of clauses and families *)
| All (* trace all clauses and families *)
val trace : string Spec -> unit (* trace clauses and families *)
val break : string Spec -> unit (* break at clauses and families *)
val detail : int ref (* 0=none, 1=default, 2=unify *)
val show : unit -> unit (* show trace, break, and detail *)
val reset : unit -> unit (* reset trace, break, and detail *)
end
structure Timers :
sig
val show : unit -> unit (* show and reset timers *)
val reset : unit -> unit (* reset timers *)
val check : unit -> unit (* display, but not no reset *)
end
structure OS :
sig
val chDir : string -> unit (* change working directory *)
val getDir : unit -> string (* get working directory *)
val exit : unit -> unit (* exit Twelf and ML *)
end
structure Compile :
sig
val optimize : bool ref (* true, optimize clauses *)
end
structure Table :
sig
datatype Strategy = Variant | Subsumption
val strategy : Strategy ref (* Variant, tabling strategy *)
val strengthen : bool ref (* false, tabling optimization *)
val top : unit -> unit (* top-level for tabled queries *)
end
structure Recon :
sig
datatype TraceMode = Progressive | Omniscient
val trace : bool ref (* false, trace term reconstruction *)
val traceMode : TraceMode ref (* Omniscient, trace mode *)
end
structure Prover :
sig
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 *)
end
val chatter : int ref (* 3, chatter level *)
val doubleCheck : bool ref (* false, check internal types *)
val unsafe : bool ref (* false, allow %assert w/o proof *)
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 queries *)
structure Config :
sig
type config (* configuration *)
val suffix : string ref (* suffix of configuration files *)
val read : string -> config (* read config file *)
val load : config -> Status (* reset and load configuration *)
val append : config -> Status (* load configuration (w/o reset) *)
val define : string list -> config (* define configuration *)
end
val make : string -> Status (* read and load configuration *)
val version : string (* Twelf version *)
end; (* signature TWELF *)
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