The AST in Nimrod
=================
This section describes how the AST is modelled with Nimrod's type system.
The AST consists of nodes (``PNimrodNode``) with a variable number of
children. Each node has a field named ``kind`` which describes what the node
contains:
.. code-block:: nimrod
type
TNimrodNodeKind = enum ## kind of a node; only explanatory
nnkNone, ## invalid node kind
nnkEmpty, ## empty node
nnkIdent, ## node contains an identifier
nnkIntLit, ## node contains an int literal (example: 10)
nnkStrLit, ## node contains a string literal (example: "abc")
nnkNilLit, ## node contains a nil literal (example: nil)
nnkCaseStmt, ## node represents a case statement
... ## many more
PNimrodNode = ref TNimrodNode
TNimrodNode {.final.} = object
case kind: TNimrodNodeKind ## the node's kind
of nnkNone, nnkEmpty, nnkNilLit:
nil ## node contains no additional fields
of nnkCharLit..nnkInt64Lit:
intVal: biggestInt ## the int literal
of nnkFloatLit..nnkFloat64Lit:
floatVal: biggestFloat ## the float literal
of nnkStrLit..nnkTripleStrLit:
strVal: string ## the string literal
of nnkIdent:
ident: TNimrodIdent ## the identifier
of nnkSym:
symbol: PNimrodSymbol ## the symbol (after symbol lookup phase)
else:
sons: seq[PNimrodNode] ## the node's sons (or children)
For the ``PNimrodNode`` type, the ``[]`` operator has been overloaded:
``n[i]`` is ``n``'s ``i``-th child.
To specify the AST for the different Nimrod constructs, the notation
``nodekind(son1, son2, ...)`` or ``nodekind(value)`` or
``nodekind(field=value)`` is used.
Some child may be missing. A missing child is a node of kind ``nnkEmpty``;
a child can never be nil.
Leaf nodes/Atoms
================
A leaf of the AST often corresponds to a terminal symbol in the concrete
syntax.
----------------- ---------------------------------------------
Nimrod expression corresponding AST
----------------- ---------------------------------------------
``42`` ``nnkIntLit(intVal = 42)``
``42'i8`` ``nnkInt8Lit(intVal = 42)``
``42'i16`` ``nnkInt16Lit(intVal = 42)``
``42'i32`` ``nnkInt32Lit(intVal = 42)``
``42'i64`` ``nnkInt64Lit(intVal = 42)``
``42.0`` ``nnkFloatLit(floatVal = 42.0)``
``42.0'f32`` ``nnkFloat32Lit(floatVal = 42.0)``
``42.0'f64`` ``nnkFloat64Lit(floatVal = 42.0)``
``"abc"`` ``nnkStrLit(strVal = "abc")``
``r"abc"`` ``nnkRStrLit(strVal = "abc")``
``"""abc"""`` ``nnkTripleStrLit(strVal = "abc")``
``' '`` ``nnkCharLit(intVal = 32)``
``nil`` ``nnkNilLit()``
``myIdentifier`` ``nnkIdent(ident = !"myIdentifier")``
``myIdentifier`` after lookup pass: ``nnkSym(symbol = ...)``
----------------- ---------------------------------------------
Identifiers are ``nnkIdent`` nodes. After the name lookup pass these nodes
get transferred into ``nnkSym`` nodes. However, a macro receives an AST that
has not been checked for semantics and thus the identifiers have not been
looked up. Macros should deal with ``nnkIdent`` nodes and do not need to deal
with ``nnkSym`` nodes.
Calls/expressions
=================
Command call
------------
Concrete syntax:
.. code-block:: nimrod
echo "abc", "xyz"
AST:
.. code-block:: nimrod
nnkCommand(nnkIdent(!"echo"), nnkStrLit("abc"), nnkStrLit("xyz"))
Call with ``()``
----------------
Concrete syntax:
.. code-block:: nimrod
echo("abc", "xyz")
AST:
.. code-block:: nimrod
nnkCall(nnkIdent(!"echo"), nnkStrLit("abc"), nnkStrLit("xyz"))
Infix operator call
-------------------
Concrete syntax:
.. code-block:: nimrod
"abc" & "xyz"
AST:
.. code-block:: nimrod
nnkInfix(nnkIdent(!"&"), nnkStrLit("abc"), nnkStrLit("xyz"))
Prefix operator call
--------------------
Concrete syntax:
.. code-block:: nimrod
? "xyz"
AST:
.. code-block:: nimrod
nnkPrefix(nnkIdent(!"?"), nnkStrLit("abc"))
Postfix operator call
---------------------
**Note:** There are no postfix operators in Nimrod. However, the
``nnkPostfix`` node is used for the *asterisk export marker* ``*``:
Concrete syntax:
.. code-block:: nimrod
identifier*
AST:
.. code-block:: nimrod
nnkPostfix(nnkIdent(!"*"), nnkIdent(!"identifier"))
Call with named arguments
-------------------------
Concrete syntax:
.. code-block:: nimrod
writeln(file=stdout, "hallo")
AST:
.. code-block:: nimrod
nnkCall(nnkIdent(!"writeln"),
nnkExprEqExpr(nnkIdent(!"file"), nnkIdent(!"stdout")),
nnkStrLit("hallo"))
Dereference operator ``^``
--------------------------
Concrete syntax:
.. code-block:: nimrod
x^
AST:
.. code-block:: nimrod
nnkDerefExpr(nnkIdent(!"x"))
Addr operator
-------------
Concrete syntax:
.. code-block:: nimrod
addr(x)
AST:
.. code-block:: nimrod
nnkAddr(nnkIdent(!"x"))
Cast operator
-------------
Concrete syntax:
.. code-block:: nimrod
cast[T](x)
AST:
.. code-block:: nimrod
nnkCast(nnkIdent(!"T"), nnkIdent(!"x"))
Object access operator ``.``
----------------------------
Concrete syntax:
.. code-block:: nimrod
x.y
AST:
.. code-block:: nimrod
nnkDotExpr(nnkIdent(!"x"), nnkIdent(!"y"))
Array access operator ``[]``
----------------------------
Concrete syntax:
.. code-block:: nimrod
x[y]
AST:
.. code-block:: nimrod
nnkBracketExpr(nnkIdent(!"x"), nnkIdent(!"y"))
Parentheses
-----------
Parentheses for affecting operator precedence or tuple construction
are built with the ``nnkPar`` node.
Concrete syntax:
.. code-block:: nimrod
(1, 2, (3))
AST:
.. code-block:: nimrod
nnkPar(nnkIntLit(1), nnkIntLit(2), nnkPar(nnkIntLit(3)))
Curly braces
------------
Curly braces are used as the set constructor.
Concrete syntax:
.. code-block:: nimrod
{1, 2, 3}
AST:
.. code-block:: nimrod
nnkCurly(nnkIntLit(1), nnkIntLit(2), nnkIntLit(3))
Brackets
--------
Brackets are used as the array constructor.
Concrete syntax:
.. code-block:: nimrod
[1, 2, 3]
AST:
.. code-block:: nimrod
nnkBracket(nnkIntLit(1), nnkIntLit(2), nnkIntLit(3))
Ranges
------
Ranges occur in set constructors, case statement branches or array slices.
Concrete syntax:
.. code-block:: nimrod
1..3
AST:
.. code-block:: nimrod
nnkRange(nnkIntLit(1), nnkIntLit(3))
If expression
-------------
The representation of the if expression is subtle, but easy to traverse.
Concrete syntax:
.. code-block:: nimrod
if cond1: expr1 elif cond2: expr2 else: expr3
AST:
.. code-block:: nimrod
nnkIfExpr(
nnkElifExpr(cond1, expr1),
nnkElifExpr(cond2, expr2),
nnkElseExpr(expr3)
)
Statements
==========
If statement
------------
The representation of the if statement is subtle, but easy to traverse. If
there is no ``else`` branch, no ``nnkElse`` child exists.
Concrete syntax:
.. code-block:: nimrod
if cond1:
stmt1
elif cond2:
stmt2
elif cond3:
stmt3
else:
stmt4
AST:
.. code-block:: nimrod
nnkIfStmt(
nnkElifBranch(cond1, stmt1),
nnkElifBranch(cond2, stmt2),
nnkElifBranch(cond3, stmt3),
nnkElse(stmt4)
)
When statement
--------------
Like the ``if`` statement, but the root has the kind ``nnkWhenStmt``.
Assignment
----------
Concrete syntax:
.. code-block:: nimrod
x = 42
AST:
.. code-block:: nimrod
nnkAsgn(nnkIdent(!"x"), nnkIntLit(42))
Statement list
--------------
Concrete syntax:
.. code-block:: nimrod
stmt1
stmt2
stmt3
AST:
.. code-block:: nimrod
nnkStmtList(stmt1, stmt2, stmt3)
Case statement
--------------
Concrete syntax:
.. code-block:: nimrod
case expr1
of expr2, expr3..expr4:
stmt1
of expr5:
stmt2
elif cond1:
stmt3
else:
stmt4
AST:
.. code-block:: nimrod
nnkCaseStmt(
expr1,
nnkOfBranch(expr2, nnkRange(expr3, expr4), stmt1),
nnkOfBranch(expr5, stmt2),
nnkElifBranch(cond1, stmt3),
nnkElse(stmt4)
)
The ``nnkElifBranch`` and ``nnkElse`` parts may be missing.
While statement
---------------
Concrete syntax:
.. code-block:: nimrod
while expr1:
stmt1
AST:
.. code-block:: nimrod
nnkWhileStmt(expr1, stmt1)
For statement
-------------
Concrete syntax:
.. code-block:: nimrod
for ident1, ident2 in expr1:
stmt1
AST:
.. code-block:: nimrod
nnkForStmt(ident1, ident2, expr1, stmt1)
Try statement
-------------
Concrete syntax:
.. code-block:: nimrod
try:
stmt1
except e1, e2:
stmt2
except e3:
stmt3
except:
stmt4
finally:
stmt5
AST:
.. code-block:: nimrod
nnkTryStmt(
stmt1,
nnkExceptBranch(e1, e2, stmt2),
nnkExceptBranch(e3, stmt3),
nnkExceptBranch(stmt4),
nnkFinally(stmt5)
)
Return statement
----------------
Concrete syntax:
.. code-block:: nimrod
return expr1
AST:
.. code-block:: nimrod
nnkReturnStmt(expr1)
Yield statement
---------------
Like ``return``, but with ``nnkYieldStmt`` kind.
Discard statement
-----------------
Like ``return``, but with ``nnkDiscardStmt`` kind.
Continue statement
------------------
Concrete syntax:
.. code-block:: nimrod
continue
AST:
.. code-block:: nimrod
nnkContinueStmt()
Var section
-----------
To be written.
Const section
-------------
To be written.
Type section
------------
To be written.
Procedure declaration
---------------------
To be written.
Iterator declaration
--------------------
To be written.
Template declaration
--------------------
To be written.
Macro declaration
-----------------
To be written.
Special node kinds
==================
There are several node kinds that are used for semantic checking or code
generation. These are accessible from this module, but should not be used.
Other node kinds are especially designed to make AST manipulations easier.
These are explained here.
To be written.