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The XL abstract syntax tree (AST)
is the internal representation of
XL0. It can be thought of as an equivalent for
XL of S-expressions for Lisp
(XL0 being the equivalent of M-expressions
which were never really used in Lisp).
The XL AST format defines 7 types of nodes, 4 leafs and 3 complex
nodes, and also "wildcard" nodes:
- Name leaf nodes hold a name such as Foo or a
symbol such as %%. A name node that wasn't generated by the
parser could contain a sequence of characters that would not be valid
according to the XL0 syntax, such as 0a-- N__.
- Integer leaf nodes contain an integral value such as
22, including based numbers such as 16#3AE (an
hexadecimal number). The size of the largest integer number that
can be represented is implementation-dependent, but the
parser should detect overflows (it doesn't at this stage).
- Real leaf nodes contain a floating-point value such as
3.14E-15, including based numbers such as 2#1.01#E3.
The precision of the representation is implementation-dependent.
- Text leaf nodes contain textual data such as
"Hello World" or 'Quoted'. The quote being used is
recorded, and can be used by the translation process to
give different semantics to different kinds of text. By convention,
"" are used for text and '' are used for characters.
Other text delimiters for multi-line text can be defined in the
xl.syntax input file
(currently, double back-quote, as in ``Hello``, but I'm
considering using what C considers shift operators, as in
<<Hello>>. Opinions?)
- Block is a complex node with one child which, in XL0, is
surrounded by delimiters, such as (A), [A] or
{A} (in all cases, the child being A). Special
delimiters I+ and I- are used internally to represent
the delimiters of indentation blocks.
- Prefix is a complex node with two children, one prefixing
the other in XL0. It will be used for not A, where the name
node not is the left child and A is the right
child.
Somewhat confusingly, this type of node is also used for postfix
operators, which are postfix only from a parsing point of view. For
instance, N! is a prefix node with N as its left
child and ! as its right child, though ! is known as
a postfix operator by the parser.
- Infix is a complex node with two children and an operator
name, the children surrounding the operator
in XL0. It will be used for A+B, where the operator is
+ and the children are two name nodes A and B.
- Wildcard nodes are named nodes used for matching
trees. For instance, if A and B are wildcards, then
you can build a tree for not A + B and it will match trees
like not (Z-5) + 2*4 or not 0 + 'A'.
The AST for the C++ version of the XL compiler was called Coda and
was a much more complicated object-oriented structure. The new AST
format makes the translation process much simpler.
See also the XL0 syntax.
To see the XL0 representation of a given input, you can use the
following command line:
./nxl -parse input.xl -style debug -show
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