[developers] Questions on the syntax of TDL

Guy Emerson gete2 at cam.ac.uk
Wed Jul 11 11:34:35 CEST 2018

To answer question 2 on the wiki, instances are formally different from

After all type definitions have been given, the type hierarchy can be
defined along with all its constraints (or fail to be defined, if there's a
conflict somewhere).  An instance is a feature structure that must follow
the type hierarchy and its constraints.  A type is given a name, and this
becomes part of the type hierarchy.  An instance is given a name, but this
is for ease of reference and doesn't affect the hierarchy.

For example, in defining a type, we are free to combine any supertypes to
create multiple inheritance in the type hierarchy.  However, in defining an
instance, we must choose which type it instantiates.  The definition of an
instance gives its type, not a supertype.  (The type's feature structure
will subsume the instance's feature structure, but both feature structures
have the same type.)  Now, in terms of syntax, there are two options:

1. An instance must be defined as instantiating one specific type (so
"multiple supertype" syntax is not allowed).

2. An instance can be defined as instantiating multiple types, in which
case the TDL processor must perform unification, either finding the glb if
it exists, or failing.

I'm not sure there's a use case for 2.  It would seem to be clearer to
directly state the type of the instance, rather than leaving it implicit.
In principle, option 2 would allow instantiating a glb type that isn't
explicitly defined... but I don't know if anyone ever has a need for that,
and if the glb is important, it would probably be clearer to define it
explicitly.  So I think option 1 is better.

For questions 3, 5, and 6:

Requiring types to come first seems sensible.  I think everyone already
does this, anyway?

Fixing the position of docstrings seems sensible.  Is there any variation
in current code?

Allowing extra whitespace and comments seems sensible.

2018-07-11 2:01 GMT+01:00 Michael Wayne Goodman <goodmami at uw.edu>:

> I attempted to define a BNF-like description of TDL syntax on the wiki:
> http://moin.delph-in.net/TdlRfc
> I tried to follow the partial BNF in the LKB source and often referred to
> the lisp code itself in order to fill out the rest of the description.
> My 3 questions above are concisely repeated at the bottom of the wiki
> along with some others.
> I welcome corrections and discussion (here or on the wiki) from any TDL
> nerds or authorities (especially if you've written a TDL parser).
> On Mon, Jul 9, 2018 at 12:49 PM, Michael Wayne Goodman <goodmami at uw.edu>
> wrote:
>> Hi developers,
>> I'm taking a closer look at the syntax of TDL files and the situation is
>> a bit of a mess. Can anyone help me clarify some things? (I'll restrict
>> myself to 3 questions for now)
>> The Copestake 2002 reference (Implementing TFS Grammars) has a BNF for
>> TDL, but it's a bit out of date and, according to comments in the LKB
>> source code, incorrect in parts. The LKB source comments are scattered,
>> incomplete, inconsistent, and also a bit outdated. There is not much on the
>> wiki. There is some discussion in the mailing list archives (much from
>> before my time in DELPH-IN), but it's not clear how current those
>> descriptions are.
>> Q1: Are supertypes special in a definition?
>> The BNF (in the LKB source) says this:
>>     Type-def -> Type { Avm-def | Subtype-def} . |
>>                          Type { Avm-def | Subtype-def}.
>>     Avm-def -> := Conjunction | Comment Conjunction
>>     Conjunction -> Term { & Term } *
>>     Term -> Type | Feature-term | Diff-list | List | Coreference
>> That makes it sound like I could do this:
>>     mytype := [ FEAT val ] & supertype.
>> or even:
>>     mytype := <! diff list.. !> & #coref & supertype.
>> But elsewhere it seems like a list of parents is special and appears
>> before the rest of the conjunction. E.g., at read-tdl-avm-def of
>> lingo/lkb/src/io-tdl/tdltypeinput.lsp I see this alternate definition of
>> Avm-def:
>>   ;;; Avm-def -> := Parents Conjunction | Parents Comment Conjunction |
>>   ;;;               Parents | Parents Comment
>> It seems that both ACE and PET are fine with putting supertypes after the
>> feature list (and some other variations). I'm fine with this, but I wonder
>> what it means for docstrings (see Q3 below), which (I think) are supposed
>> to appear after the list of parents and before the feature list.
>> Q2: Subtype-def is now just a variant of Avm-def, yes?
>> The BNF still describes subtyping (with the :< operator) as only taking a
>> single parent:
>>     Subtype-def ->  :< type
>> But I believe the consensus is that this is unnecessary (it's equivalent
>> to using := with only a supertype), so :< is treated as equivalent to :=
>> (to avoid breaking backward compatibility). Is this interpretation used by
>> all processors?
>> Q3: What's the final word with type comments / docstrings?
>> I find evidence of 3 proposed variants: (1) a block of ";" comments
>> before a typename (LTDB-style); (2) a block of ";" comments within a type
>> description; and (3) a "doc string" within a type description. Furthermore,
>> there is a question as to whether comments or strings within a type go
>> after the ":=" or after the list of supertypes. I think #| ... |# comments
>> were not considered for this purpose.
>> My guess is this:
>> * LTDB-style comments (before the type identifier) are processed
>> separately from TDL-parsing
>> * type-internal comments can go anywhere but are discarded
>> * type-internal doc strings must appear after the list of supertypes and
>> are later available for inspection (they are included as a non-functional
>> part of a type)
>> ACE seems happy with my assumptions, although PET doesn't seem to like
>> doc strings at all.
>> Thanks!
>> --
>> Michael Wayne Goodman
> --
> Michael Wayne Goodman
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