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- Current State of Exceptions in Rakudo and Perl 6
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- Perl 6 Hackathon in Oslo: Be Prepared!
- Localization for Exception Messages
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Sun, 31 Mar 2013
Rakudo's Abstract Syntax Tree
After or while a compiler parses a program, the compiler usually translates the source code into a tree format called Abstract Syntax Tree, or AST for short.
The optimizer works on this program representation, and then the code generation stage turns it into a format that the platform underneath it can understand. Actually I wanted to write about the optimizer, but noticed that understanding the AST is crucial to understanding the optimizer, so let's talk about the AST first.
The Rakudo Perl 6 Compiler uses an AST
format called QAST. QAST nodes derive from the common superclass
QAST::Node, which sets up the basic structure of all QAST
classes. Each QAST node has a list of child nodes, possibly a hash map for
unstructured annotations, an attribute (confusingly) named
for storing the lower-level parse tree (which is used to extract line numbers
and context), and a bit of extra infrastructure.
The most important node classes are the following:
- A list of statements. Each child of the node is considered a separate statement.
- A single operation that usually maps to a primitive operation of the underlying platform, like adding two integers, or calling a routine.
- QAST::IVal, QAST::NVal, QAST::SVal
- Those hold integer, float ("numeric") and string constants respectively.
- Holds a reference to a more complex object (for example a class) which is serialized separately.
- A list of statements that introduces a separate lexical scope.
- A variable
- A node that can evaluate to different child nodes, depending on the context it is compiled it.
To give you a bit of a feel of how those node types interact, I want to give a few examples of Perl 6 examples, and what AST they could produce. (It turns out that Perl 6 is quite a complex language under the hood, and usually produces a more complicated AST than the obvious one; I'll ignore that for now, in order to introduce you to the basics.)
Ops and Constants
23 + 42 could, in the simplest case, produce
QAST::Op.new( :op('add'), QAST::IVal.new(:value(23)), QAST::IVal.new(:value(42)), );
QAST::Op encodes a primitive operation, an addition of
two numbers. The
:op argument specifies which operation to use.
The child nodes are two constants, both of type
hold the operands of the low-level operation
Now the low-level
add operation is not polymorphic, it always
adds two floating-point values, and the result is a floating-point value
again. Since the arguments are integers and not floating point values, they
are automatically converted to float first. That's not the desired semantics for Perl 6; actually the operator
+ is implemented as a subroutine of name
&infix:<+>, so the real generated code is closer to
QAST::Op.new( :op('call'), :name('&infix:<+>'), # name of the subroutine to call QAST::IVal.new(:value(23)), QAST::IVal.new(:value(42)), );
Variables and Blocks
Using a variable is as simple as writing
QAST::Var.new(:name('name-of-the-variable')), but it must be declared
first. This is done with
But there is a slight caveat: in Perl 6 a variable is always scoped to a
block. So while you can't ordinarily mention a variable prior to its
declaration, there are indirect ways to achieve that (lookup by name, and
eval(), to name just two).
So in Rakudo there is a convention to create
QAST::Stmts children. The first holds all the
declarations, and the second all the actual code. That way all the declaration
always come before the rest of the code.
my $x = 42; say $x compiles to roughly this:
QAST::Block.new( QAST::Stmts.new( QAST::Var.new(:name('$x'), :decl('var'), :scope('lexical')), ), QAST::Stmts.new( QAST::Op.new( :op('p6store'), QAST::Var.new(:name('$x')), QAST::IVal.new(:value(42)), ), QAST::Op.new( :op('call'), :name('&say'), QAST::Var.new(:name('$x')), ), ), );
Polymorphism and QAST::Want
Perl 6 distinguishes between native types and reference types. Native types are closer to the machine, and their type name is always lower case in Perl 6.
Integer literals are polymorphic in that they can be either a native
int or a "boxed" reference type
To model this in the AST,
QAST::Want nodes can contain
multiple child nodes. The compile-time context decides which of those is
So the integer literal
42 actually produces not just a simple
QAST::IVal node but rather this:
QAST::Want.new( QAST::WVal(Int.new(42)), 'Ii', QAST::Ival(42), )
Int.new(42) is just a nice notation to indicate a
boxed integer object; it doesn't quite work like this in the code that
translate Perl 6 source code into ASTs).
The first child of a
QAST::Want node is the one used by
default, if no other alternative matches. The comes a list where the elements
with odd indexes are format specifications (here
integers) and the elements at even-side indexes are the AST to use in that
An interesting format specification is
'v' for void context,
which is always chosen when the return value from the current expression isn't
used at all. In Perl 6 this is used to eagerly evaluate lazy lists that are
used in void context, and for several optimizations.
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