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fixes #1135

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Araq 2014-07-01 01:21:26 +02:00
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doc/trmacros.txt
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@ -1,275 +0,0 @@
=========================================================
Term rewriting macros for Nimrod
=========================================================
:Author: Andreas Rumpf
Term rewriting macros are macros or templates that have not only a *name* but
also a *pattern* that is searched for after the semantic checking phase of
the compiler: This means they provide an easy way to enhance the compilation
pipeline with user defined optimizations:
.. code-block:: nimrod
template optMul{`*`(a, 2)}(a: int): int = a+a
let x = 3
echo x * 2
The compiler now rewrites ``x * 2`` as ``x + x``. The code inside the
curlies is the pattern to match against. The operators ``*``, ``**``,
``|``, ``~`` have a special meaning in patterns if they are written in infix
notation, so to match verbatim against ``*`` the ordinary function call syntax
needs to be used.
Unfortunately optimizations are hard to get right and even the tiny example
is **wrong**:
.. code-block:: nimrod
template optMul{`*`(a, 2)}(a: int): int = a+a
proc f(): int =
echo "side effect!"
result = 55
echo f() * 2
We cannot duplicate 'a' if it denotes an expression that has a side effect!
Fortunately Nimrod supports side effect analysis:
.. code-block:: nimrod
template optMul{`*`(a, 2)}(a: int{noSideEffect}): int = a+a
proc f(): int =
echo "side effect!"
result = 55
echo f() * 2 # not optimized ;-)
So what about ``2 * a``? We should tell the compiler ``*`` is commutative. We
cannot really do that however as the following code only swaps arguments
blindly:
.. code-block:: nimrod
template mulIsCommutative{`*`(a, b)}(a, b: int): int = b*a
What optimizers really need to do is a *canonicalization*:
.. code-block:: nimrod
template canonMul{`*`(a, b)}(a: int{lit}, b: int): int = b*a
The ``int{lit}`` parameter pattern matches against an expression of
type ``int``, but only if it's a literal.
Parameter constraints
=====================
The parameter constraint expression can use the operators ``|`` (or),
``&`` (and) and ``~`` (not) and the following predicates:
=================== =====================================================
Predicate Meaning
=================== =====================================================
``atom`` The matching node has no children.
``lit`` The matching node is a literal like "abc", 12.
``sym`` The matching node must be a symbol (a bound
identifier).
``ident`` The matching node must be an identifier (an unbound
identifier).
``call`` The matching AST must be a call/apply expression.
``lvalue`` The matching AST must be an lvalue.
``sideeffect`` The matching AST must have a side effect.
``nosideeffect`` The matching AST must have no side effect.
``param`` A symbol which is a parameter.
``genericparam`` A symbol which is a generic parameter.
``module`` A symbol which is a module.
``type`` A symbol which is a type.
``var`` A symbol which is a variable.
``let`` A symbol which is a ``let`` variable.
``const`` A symbol which is a constant.
``result`` The special ``result`` variable.
``proc`` A symbol which is a proc.
``method`` A symbol which is a method.
``iterator`` A symbol which is an iterator.
``converter`` A symbol which is a converter.
``macro`` A symbol which is a macro.
``template`` A symbol which is a template.
``field`` A symbol which is a field in a tuple or an object.
``enumfield`` A symbol which is a field in an enumeration.
``forvar`` A for loop variable.
``label`` A label (used in ``block`` statements).
``nk*`` The matching AST must have the specified kind.
(Example: ``nkIfStmt`` denotes an ``if`` statement.)
``alias`` States that the marked parameter needs to alias
with *some* other parameter.
``noalias`` States that *every* other parameter must not alias
with the marked parameter.
=================== =====================================================
The ``alias`` and ``noalias`` predicates refer not only to the matching AST,
but also to every other bound parameter; syntactially they need to occur after
the ordinary AST predicates:
.. code-block:: nimrod
template ex{a = b + c}(a: int{noalias}, b, c: int) =
# this transformation is only valid if 'b' and 'c' do not alias 'a':
a = b
inc a, b
Pattern operators
=================
The operators ``*``, ``**``, ``|``, ``~`` have a special meaning in patterns
if they are written in infix notation.
The ``|`` operator
------------------
The ``|`` operator if used as infix operator creates an ordered choice:
.. code-block:: nimrod
template t{0|1}(): expr = 3
let a = 1
# outputs 3:
echo a
The matching is performed after the compiler performed some optimizations like
constant folding, so the following does not work:
.. code-block:: nimrod
template t{0|1}(): expr = 3
# outputs 1:
echo 1
The reason is that the compiler already transformed the 1 into "1" for
the ``echo`` statement. However, a term rewriting macro should not change the
semantics anyway. In fact they can be deactived with the ``--patterns:off``
command line option or temporarily with the ``patterns`` pragma.
The ``{}`` operator
-------------------
A pattern expression can be bound to a pattern parameter via the ``expr{param}``
notation:
.. code-block:: nimrod
template t{(0|1|2){x}}(x: expr): expr = x+1
let a = 1
# outputs 2:
echo a
The ``~`` operator
------------------
The ``~`` operator is the **not** operator in patterns:
.. code-block:: nimrod
template t{x = (~x){y} and (~x){z}}(x, y, z: bool): stmt =
x = y
if x: x = z
var
a = false
b = true
c = false
a = b and c
echo a
The ``*`` operator
------------------
The ``*`` operator can *flatten* a nested binary expression like ``a & b & c``
to ``&(a, b, c)``:
.. code-block:: nimrod
var
calls = 0
proc `&&`(s: varargs[string]): string =
result = s[0]
for i in 1..len(s)-1: result.add s[i]
inc calls
template optConc{ `&&` * a }(a: string): expr = &&a
let space = " "
echo "my" && (space & "awe" && "some " ) && "concat"
# check that it's been optimized properly:
doAssert calls == 1
The second operator of `*` must be a parameter; it is used to gather all the
arguments. The expression ``"my" && (space & "awe" && "some " ) && "concat"``
is passed to ``optConc`` in ``a`` as a special list (of kind ``nkArgList``)
which is flattened into a call expression; thus the invocation of ``optConc``
produces:
.. code-block:: nimrod
`&&`("my", space & "awe", "some ", "concat")
The ``**`` operator
-------------------
The ``**`` is much like the ``*`` operator, except that it gathers not only
all the arguments, but also the matched operators in reverse polish notation:
.. code-block:: nimrod
import macros
type
TMatrix = object
dummy: int
proc `*`(a, b: TMatrix): TMatrix = nil
proc `+`(a, b: TMatrix): TMatrix = nil
proc `-`(a, b: TMatrix): TMatrix = nil
proc `$`(a: TMatrix): string = result = $a.dummy
proc mat21(): TMatrix =
result.dummy = 21
macro optM{ (`+`|`-`|`*`) ** a }(a: TMatrix): expr =
echo treeRepr(a)
result = newCall(bindSym"mat21")
var x, y, z: TMatrix
echo x + y * z - x
This passes the expression ``x + y * z - x`` to the ``optM`` macro as
an ``nnkArgList`` node containing::
Arglist
Sym "x"
Sym "y"
Sym "z"
Sym "*"
Sym "+"
Sym "x"
Sym "-"
(Which is the reverse polish notation of ``x + y * z - x``.)
Parameters
==========
Parameters in a pattern are type checked in the matching process. If a
parameter is of the type ``varargs`` it is treated specially and it can match
0 or more arguments in the AST to be matched against:
.. code-block:: nimrod
template optWrite{
write(f, x)
((write|writeln){w})(f, y)
}(x, y: varargs[expr], f: TFile, w: expr) =
w(f, x, y)

2
web/nimrod.ini
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@ -36,7 +36,7 @@ UNIX. We don't believe this to be a coincidence. - Jeremy S. Anderson."""
[Documentation] [Documentation]
doc: "endb;intern;apis;lib;manual;tut1;tut2;nimrodc;overview;filters;trmacros" doc: "endb;intern;apis;lib;manual;tut1;tut2;nimrodc;overview;filters"
doc: "tools;c2nim;niminst;nimgrep;gc;estp;idetools;docgen;koch;backends.txt" doc: "tools;c2nim;niminst;nimgrep;gc;estp;idetools;docgen;koch;backends.txt"
pdf: "manual;lib;tut1;tut2;nimrodc;c2nim;niminst;gc" pdf: "manual;lib;tut1;tut2;nimrodc;c2nim;niminst;gc"
srcdoc2: "system.nim;impure/graphics;wrappers/sdl" srcdoc2: "system.nim;impure/graphics;wrappers/sdl"