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1.1 New Features

The current version 1.4 from December 27, 2002 incorporates the following major changes from Twelf 1.3 from September 13, 2000.

World Checking (see section Regular Worlds).

World checking verifies regularity of the parameters and hypothesis that can be introduced by terms in a signature. The new declarations are %block and %worlds. This formally checks part of the adequacy theorem that is usually left implicit in the encoding and is is used centrally by the coverage checker.

Coverage and Totality Checking (see section Coverage, see section Totality).

Proofs of meta-theorems given in relational form can now be verified if they are of order 2 or less. The new relevant declarations are %covers and %total. The former can also be used to check that sets of patterns in the arguments of a type family are exhaustive.

Mode Checking (see section Modes)

Mode checking has been extended so that multiple modes can be checked for the same predicate, even though not simultaneously. This allows certain relations to serve as proofs for biconditional meta-theorems. Also, some predicates in constraint domains can have multiple modes simultaneously in effect.

Tabled Search (see section Tabled Logic Programming)

An experimental logic programming engine for tabled logic programming is available in this release. The corresponding declarations are %tabled a (to declare a type family to be tabled) and %querytabled to start tabled search.

Deterministic Search (see section Deterministic Type Families)

Type families can be declared deterministic for search, which means that after the first solution has been found, backtracking will not find any further solutions. The relevant declaration is %deterministic a.

Family-Level Definitions (see section Clause Definitions)

Twelf now permits family-level definitions that are opaque to logic programming execution. However, not all aspects of the present release handle them properly, so they should be considered an experimental feature. Furthermore, defined constants will now be used for logic programming search when prefixed with %clause.

Tracing Term Reconstruction (see section Tracing Reconstruction)

Term reconstruction can print each typing judgment it establishes in order to help diagnose subtle type errors or ambiguities.

Portability (see section Installation)

Twelf has been ported to be compliant with the Definition of Standard ML in its 1997 revision. As a result, it now supports Poly/ML and MLton in addition to Standard ML of New Jersey (SML/NJ).


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1.2 Quick Start

Assuming you are running on a Unix system with SML of New Jersey 110.0.3 already installed (see section Installation) you can build Twelf as follows. Here ‘%’ is assumed to be the shell prompt. You may need to edit the file ‘Makefile’ to give the proper location for sml.

% gunzip twelf-1-4.tar.gz
% tar -xf twelf-1-4.tar
% cd twelf
% make
% bin/twelf-server
Twelf 1.4, Dec 27 2002
%% OK %%

For SML/NJ version 110.20 or greater, use make -f smlnj/Makefile. For Poly/ML use make -f polyml/Makefile. For MLton use make -f mlton/Makefile.

You can now load the examples used in this guide and pose an example query as shown below. The prompt from the Twelf top-level is ‘?-’. To drop from the Twelf top-level to the ML top-level, type C-c (<CTRL> c). To exit the Twelf server you may issue the quit command or type C-d (<CTRL> c).

make examples/guide/sources.cfg
top
?- of (lam [x] x) T.
Solving...
T = arrow T1 T1.
More? y
No more solutions
?- C-c
interrupt
%% OK %%
quit
%

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