TypeAddendum < Main

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---+Grammar Engineering Frequently Asked Questions

---++What is a type addendum statement, and when should I use one?

Type addendum statements are a recent extenion to tdl syntax
(as of 5/2005, LKB only, not yet interpreted by PET).  Type
addendum statements allow you add information to a previously
defined type.  Prior to the development of the Matrix, there wasn't
really a need for this kind of functionality: if you wanted to 
add to a type, you could just edit its defintion.  However, we
strongly discourage editing the matrix files (particularly
=matrix.tdl=) when building a grammar from the Matrix.

You may find that you would like to add a feature to an existing
type (notably the values of =HEAD=), or add a constraint to an
existing type.  To some extent, this can be done by adding subtypes,
but this can become awkward.

A type addendum statement looks like this:

<pre>
existing-type :+ new-supertype & 
"documentation string"
   [ EXISTING-FEATURE more-specific-value,
     NEW-FEATURE some-value ].
</pre>

Unlike type definitions, type addendum statements use <tt>:+</tt>
instead of <tt>:=</tt>.  Type addendum statements are only valid
with existing types.  There must be at least one piece of information
after the <tt>:+</tt>: a new supertype, a documentation string (in quotes),
or a constraint (in square brackets), or any combination of those.
Note that it's not valid to specify a supertype in a type addendum
which is already a supertype of the type in question.

In general, if you would like to add information to a type already
declared in the Matrix, a type addendum statement is the way to go.
For types that are defined in language-specific files, it's probably
best to avoid type addenda.

---+++Related topics

   * [[TdlSyntax][What do the punctuation marks mean in the tdl files? (A very basic guide to tdl syntax.)]]
   * [[NewFeature][I'm trying to add a new feature, but the LKB doesn't like it. What should I do?]]
  
---

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-- Main.EmilyBender - 18 May 2005
Topic revision: r1 - 2005-05-19 - EmilyBender