trusty (3) ms_transform.3erl.gz
NAME
ms_transform - Parse_transform that translates fun syntax into match specifications.
DESCRIPTION
This module implements the parse_transform that makes calls to ets and dbg:fun2ms/1 translate into
literal match specifications. It also implements the back end for the same functions when called from the
Erlang shell.
The translations from fun's to match_specs is accessed through the two "pseudo functions" ets:fun2ms/1
and dbg:fun2ms/1.
Actually this introduction is more or less an introduction to the whole concept of match specifications.
Since everyone trying to use ets:select or dbg seems to end up reading this page, it seems in good place
to explain a little more than just what this module does.
There are some caveats one should be aware of, please read through the whole manual page if it's the
first time you're using the transformations.
Match specifications are used more or less as filters. They resemble usual Erlang matching in a list
comprehension or in a fun used in conjunction with lists:foldl etc. The syntax of pure match
specifications is somewhat awkward though, as they are made up purely by Erlang terms and there is no
syntax in the language to make the match specifications more readable.
As the match specifications execution and structure is quite like that of a fun, it would for most
programmers be more straight forward to simply write it using the familiar fun syntax and having that
translated into a match specification automatically. Of course a real fun is more powerful than the match
specifications allow, but bearing the match specifications in mind, and what they can do, it's still more
convenient to write it all as a fun. This module contains the code that simply translates the fun syntax
into match_spec terms.
Let's start with an ets example. Using ets:select and a match specification, one can filter out rows of a
table and construct a list of tuples containing relevant parts of the data in these rows. Of course one
could use ets:foldl instead, but the select call is far more efficient. Without the translation, one has
to struggle with writing match specifications terms to accommodate this, or one has to resort to the less
powerful ets:match(_object) calls, or simply give up and use the more inefficient method of ets:foldl.
Using the ets:fun2ms transformation, a ets:select call is at least as easy to write as any of the
alternatives.
As an example, consider a simple table of employees:
-record(emp, {empno, %Employee number as a string, the key
surname, %Surname of the employee
givenname, %Given name of employee
dept, %Department one of {dev,sales,prod,adm}
empyear}). %Year the employee was employed
We create the table using:
ets:new(emp_tab,[{keypos,#emp.empno},named_table,ordered_set]).
Let's also fill it with some randomly chosen data for the examples:
[{emp,"011103","Black","Alfred",sales,2000},
{emp,"041231","Doe","John",prod,2001},
{emp,"052341","Smith","John",dev,1997},
{emp,"076324","Smith","Ella",sales,1995},
{emp,"122334","Weston","Anna",prod,2002},
{emp,"535216","Chalker","Samuel",adm,1998},
{emp,"789789","Harrysson","Joe",adm,1996},
{emp,"963721","Scott","Juliana",dev,2003},
{emp,"989891","Brown","Gabriel",prod,1999}]
Now, the amount of data in the table is of course to small to justify complicated ets searches, but on
real tables, using select to get exactly the data you want will increase efficiency remarkably.
Lets say for example that we'd want the employee numbers of everyone in the sales department. One might
use ets:match in such a situation:
1> ets:match(emp_tab, {'_', '$1', '_', '_', sales, '_'}).
[["011103"],["076324"]]
Even though ets:match does not require a full match specification, but a simpler type, it's still
somewhat unreadable, and one has little control over the returned result, it's always a list of lists.
OK, one might use ets:foldl or ets:foldr instead:
ets:foldr(fun(#emp{empno = E, dept = sales},Acc) -> [E | Acc];
(_,Acc) -> Acc
end,
[],
emp_tab).
Running that would result in ["011103","076324"] , which at least gets rid of the extra lists. The fun is
also quite straightforward, so the only problem is that all the data from the table has to be transferred
from the table to the calling process for filtering. That's inefficient compared to the ets:match call
where the filtering can be done "inside" the emulator and only the result is transferred to the process.
Remember that ets tables are all about efficiency, if it wasn't for efficiency all of ets could be
implemented in Erlang, as a process receiving requests and sending answers back. One uses ets because one
wants performance, and therefore one wouldn't want all of the table transferred to the process for
filtering. OK, let's look at a pure ets:select call that does what the ets:foldr does:
ets:select(emp_tab,[{#emp{empno = '$1', dept = sales, _='_'},[],['$1']}]).
Even though the record syntax is used, it's still somewhat hard to read and even harder to write. The
first element of the tuple, #emp{empno = '$1', dept = sales, _='_'} tells what to match, elements not
matching this will not be returned at all, as in the ets:match example. The second element, the empty
list is a list of guard expressions, which we need none, and the third element is the list of expressions
constructing the return value (in ets this almost always is a list containing one single term). In our
case '$1' is bound to the employee number in the head (first element of tuple), and hence it is the
employee number that is returned. The result is ["011103","076324"], just as in the ets:foldr example,
but the result is retrieved much more efficiently in terms of execution speed and memory consumption.
We have one efficient but hardly readable way of doing it and one inefficient but fairly readable (at
least to the skilled Erlang programmer) way of doing it. With the use of ets:fun2ms, one could have
something that is as efficient as possible but still is written as a filter using the fun syntax:
-include_lib("stdlib/include/ms_transform.hrl").
% ...
ets:select(emp_tab, ets:fun2ms(
fun(#emp{empno = E, dept = sales}) ->
E
end)).
This may not be the shortest of the expressions, but it requires no special knowledge of match
specifications to read. The fun's head should simply match what you want to filter out and the body
returns what you want returned. As long as the fun can be kept within the limits of the match
specifications, there is no need to transfer all data of the table to the process for filtering as in the
ets:foldr example. In fact it's even easier to read then the ets:foldr example, as the select call in
itself discards anything that doesn't match, while the fun of the foldr call needs to handle both the
elements matching and the ones not matching.
It's worth noting in the above ets:fun2ms example that one needs to include ms_transform.hrl in the
source code, as this is what triggers the parse transformation of the ets:fun2ms call to a valid match
specification. This also implies that the transformation is done at compile time (except when called from
the shell of course) and therefore will take no resources at all in runtime. So although you use the more
intuitive fun syntax, it gets as efficient in runtime as writing match specifications by hand.
Let's look at some more ets examples. Let's say one wants to get all the employee numbers of any employee
hired before the year 2000. Using ets:match isn't an alternative here as relational operators cannot be
expressed there. Once again, an ets:foldr could do it (slowly, but correct):
ets:foldr(fun(#emp{empno = E, empyear = Y},Acc) when Y < 2000 -> [E | Acc];
(_,Acc) -> Acc
end,
[],
emp_tab).
The result will be ["052341","076324","535216","789789","989891"], as expected. Now the equivalent
expression using a handwritten match specification would look something like this:
ets:select(emp_tab,[{#emp{empno = '$1', empyear = '$2', _='_'},
[{'<', '$2', 2000}],
['$1']}]).
This gives the same result, the [{'<', '$2', 2000}] is in the guard part and therefore discards anything
that does not have a empyear (bound to '$2' in the head) less than 2000, just as the guard in the foldl
example. Lets jump on to writing it using ets:fun2ms
-include_lib("stdlib/include/ms_transform.hrl").
% ...
ets:select(emp_tab, ets:fun2ms(
fun(#emp{empno = E, empyear = Y}) when Y < 2000 ->
E
end)).
Obviously readability is gained by using the parse transformation.
I'll show some more examples without the tiresome comparing-to-alternatives stuff. Let's say we'd want
the whole object matching instead of only one element. We could of course assign a variable to every part
of the record and build it up once again in the body of the fun, but it's easier to do like this:
ets:select(emp_tab, ets:fun2ms(
fun(Obj = #emp{empno = E, empyear = Y})
when Y < 2000 ->
Obj
end)).
Just as in ordinary Erlang matching, you can bind a variable to the whole matched object using a "match
in then match", i.e. a =. Unfortunately this is not general in fun's translated to match specifications,
only on the "top level", i.e. matching the whole object arriving to be matched into a separate variable,
is it allowed. For the one's used to writing match specifications by hand, I'll have to mention that the
variable A will simply be translated into '$_'. It's not general, but it has very common usage, why it is
handled as a special, but useful, case. If this bothers you, the pseudo function object also returns the
whole matched object, see the part about caveats and limitations below.
Let's do something in the fun's body too: Let's say that someone realizes that there are a few people
having an employee number beginning with a zero (0), which shouldn't be allowed. All those should have
their numbers changed to begin with a one (1) instead and one wants the list [{<Old empno>,<New empno>}]
created:
ets:select(emp_tab, ets:fun2ms(
fun(#emp{empno = [$0 | Rest] }) ->
{[$0|Rest],[$1|Rest]}
end)).
As a matter of fact, this query hits the feature of partially bound keys in the table type ordered_set,
so that not the whole table need be searched, only the part of the table containing keys beginning with 0
is in fact looked into.
The fun of course can have several clauses, so that if one could do the following: For each employee, if
he or she is hired prior to 1997, return the tuple {inventory, <employee number>}, for each hired 1997 or
later, but before 2001, return {rookie, <employee number>}, for all others return {newbie, <employee
number>}. All except for the ones named Smith as they would be affronted by anything other than the tag
guru and that is also what's returned for their numbers; {guru, <employee number>}:
ets:select(emp_tab, ets:fun2ms(
fun(#emp{empno = E, surname = "Smith" }) ->
{guru,E};
(#emp{empno = E, empyear = Y}) when Y < 1997 ->
{inventory, E};
(#emp{empno = E, empyear = Y}) when Y > 2001 ->
{newbie, E};
(#emp{empno = E, empyear = Y}) -> % 1997 -- 2001
{rookie, E}
end)).
The result will be:
[{rookie,"011103"},
{rookie,"041231"},
{guru,"052341"},
{guru,"076324"},
{newbie,"122334"},
{rookie,"535216"},
{inventory,"789789"},
{newbie,"963721"},
{rookie,"989891"}]
and so the Smith's will be happy...
So, what more can you do? Well, the simple answer would be; look in the documentation of match
specifications in ERTS users guide. However let's briefly go through the most useful "built in functions"
that you can use when the fun is to be translated into a match specification by ets:fun2ms (it's worth
mentioning, although it might be obvious to some, that calling other functions than the one's allowed in
match specifications cannot be done. No "usual" Erlang code can be executed by the fun being translated
by fun2ms, the fun is after all limited exactly to the power of the match specifications, which is
unfortunate, but the price one has to pay for the execution speed of an ets:select compared to
ets:foldl/foldr).
The head of the fun is obviously a head matching (or mismatching) one parameter, one object of the table
we select from. The object is always a single variable (can be _) or a tuple, as that's what's in ets,
dets and mnesia tables (the match specification returned by ets:fun2ms can of course be used with
dets:select and mnesia:select as well as with ets:select). The use of = in the head is allowed (and
encouraged) on the top level.
The guard section can contain any guard expression of Erlang. Even the "old" type test are allowed on the
toplevel of the guard (integer(X) instead of is_integer(X)). As the new type tests (the is_ tests) are in
practice just guard bif's they can also be called from within the body of the fun, but so they can in
ordinary Erlang code. Also arithmetics is allowed, as well as ordinary guard bif's. Here's a list of
bif's and expressions:
* The type tests: is_atom, is_float, is_integer, is_list, is_number, is_pid, is_port, is_reference,
is_tuple, is_binary, is_function, is_record
* The boolean operators: not, and, or, andalso, orelse
* The relational operators: >, >=, <, =<, =:=, ==, =/=, /=
* Arithmetics: +, -, *, div, rem
* Bitwise operators: band, bor, bxor, bnot, bsl, bsr
* The guard bif's: abs, element, hd, length, node, round, size, tl, trunc, self
* The obsolete type test (only in guards): atom, float, integer, list, number, pid, port, reference,
tuple, binary, function, record
Contrary to the fact with "handwritten" match specifications, the is_record guard works as in ordinary
Erlang code.
Semicolons (;) in guards are allowed, the result will be (as expected) one "match_spec-clause" for each
semicolon-separated part of the guard. The semantics being identical to the Erlang semantics.
The body of the fun is used to construct the resulting value. When selecting from tables one usually just
construct a suiting term here, using ordinary Erlang term construction, like tuple parentheses, list
brackets and variables matched out in the head, possibly in conjunction with the occasional constant.
Whatever expressions are allowed in guards are also allowed here, but there are no special functions
except object and bindings (see further down), which returns the whole matched object and all known
variable bindings respectively.
The dbg variants of match specifications have an imperative approach to the match specification body, the
ets dialect hasn't. The fun body for ets:fun2ms returns the result without side effects, and as matching
(=) in the body of the match specifications is not allowed (for performance reasons) the only thing left,
more or less, is term construction...
Let's move on to the dbg dialect, the slightly different match specifications translated by dbg:fun2ms.
The same reasons for using the parse transformation applies to dbg, maybe even more so as filtering using
Erlang code is simply not a good idea when tracing (except afterwards, if you trace to file). The concept
is similar to that of ets:fun2ms except that you usually use it directly from the shell (which can also
be done with ets:fun2ms).
Let's manufacture a toy module to trace on
-module(toy).
-export([start/1, store/2, retrieve/1]).
start(Args) ->
toy_table = ets:new(toy_table,Args).
store(Key, Value) ->
ets:insert(toy_table,{Key,Value}).
retrieve(Key) ->
[{Key, Value}] = ets:lookup(toy_table,Key),
Value.
During model testing, the first test bails out with a {badmatch,16} in {toy,start,1}, why?
We suspect the ets call, as we match hard on the return value, but want only the particular new call with
toy_table as first parameter. So we start a default tracer on the node:
1> dbg:tracer().
{ok,<0.88.0>}
And so we turn on call tracing for all processes, we are going to make a pretty restrictive trace
pattern, so there's no need to call trace only a few processes (it usually isn't):
2> dbg:p(all,call).
{ok,[{matched,nonode@nohost,25}]}
It's time to specify the filter. We want to view calls that resemble ets:new(toy_table,<something>):
3> dbg:tp(ets,new,dbg:fun2ms(fun([toy_table,_]) -> true end)).
{ok,[{matched,nonode@nohost,1},{saved,1}]}
As can be seen, the fun's used with dbg:fun2ms takes a single list as parameter instead of a single
tuple. The list matches a list of the parameters to the traced function. A single variable may also be
used of course. The body of the fun expresses in a more imperative way actions to be taken if the fun
head (and the guards) matches. I return true here, but it's only because the body of a fun cannot be
empty, the return value will be discarded.
When we run the test of our module now, we get the following trace output:
(<0.86.0>) call ets:new(toy_table,[ordered_set])
Let's play we haven't spotted the problem yet, and want to see what ets:new returns. We do a slightly
different trace pattern:
4> dbg:tp(ets,new,dbg:fun2ms(fun([toy_table,_]) -> return_trace() end)).
Resulting in the following trace output when we run the test:
(<0.86.0>) call ets:new(toy_table,[ordered_set])
(<0.86.0>) returned from ets:new/2 -> 24
The call to return_trace, makes a trace message appear when the function returns. It applies only to the
specific function call triggering the match specification (and matching the head/guards of the match
specification). This is the by far the most common call in the body of a dbg match specification.
As the test now fails with {badmatch,24}, it's obvious that the badmatch is because the atom toy_table
does not match the number returned for an unnamed table. So we spotted the problem, the table should be
named and the arguments supplied by our test program does not include named_table. We rewrite the start
function to:
start(Args) ->
toy_table = ets:new(toy_table,[named_table |Args]).
And with the same tracing turned on, we get the following trace output:
(<0.86.0>) call ets:new(toy_table,[named_table,ordered_set])
(<0.86.0>) returned from ets:new/2 -> toy_table
Very well. Let's say the module now passes all testing and goes into the system. After a while someone
realizes that the table toy_table grows while the system is running and that for some reason there are a
lot of elements with atom's as keys. You had expected only integer keys and so does the rest of the
system. Well, obviously not all of the system. You turn on call tracing and try to see calls to your
module with an atom as the key:
1> dbg:tracer().
{ok,<0.88.0>}
2> dbg:p(all,call).
{ok,[{matched,nonode@nohost,25}]}
3> dbg:tpl(toy,store,dbg:fun2ms(fun([A,_]) when is_atom(A) -> true end)).
{ok,[{matched,nonode@nohost,1},{saved,1}]}
We use dbg:tpl here to make sure to catch local calls (let's say the module has grown since the smaller
version and we're not sure this inserting of atoms is not done locally...). When in doubt always use
local call tracing.
Let's say nothing happens when we trace in this way. Our function is never called with these parameters.
We make the conclusion that someone else (some other module) is doing it and we realize that we must
trace on ets:insert and want to see the calling function. The calling function may be retrieved using the
match specification function caller and to get it into the trace message, one has to use the match spec
function message. The filter call looks like this (looking for calls to ets:insert):
4> dbg:tpl(ets,insert,dbg:fun2ms(fun([toy_table,{A,_}]) when is_atom(A) ->
message(caller())
end)).
{ok,[{matched,nonode@nohost,1},{saved,2}]}
The caller will now appear in the "additional message" part of the trace output, and so after a while,
the following output comes:
(<0.86.0>) call ets:insert(toy_table,{garbage,can}) ({evil_mod,evil_fun,2})
You have found out that the function evil_fun of the module evil_mod, with arity 2, is the one causing
all this trouble.
This was just a toy example, but it illustrated the most used calls in match specifications for dbg The
other, more esotheric calls are listed and explained in the Users guide of the ERTS application, they
really are beyond the scope of this document.
To end this chatty introduction with something more precise, here follows some parts about caveats and
restrictions concerning the fun's used in conjunction with ets:fun2ms and dbg:fun2ms:
Warning:
To use the pseudo functions triggering the translation, one has to include the header file
ms_transform.hrl in the source code. Failure to do so will possibly result in runtime errors rather than
compile time, as the expression may be valid as a plain Erlang program without translation.
Warning:
The fun has to be literally constructed inside the parameter list to the pseudo functions. The fun cannot
be bound to a variable first and then passed to ets:fun2ms or dbg:fun2ms, i.e this will work:
ets:fun2ms(fun(A) -> A end) but not this: F = fun(A) -> A end, ets:fun2ms(F). The later will result in a
compile time error if the header is included, otherwise a runtime error. Even if the later construction
would ever appear to work, it really doesn't, so don't ever use it.
Several restrictions apply to the fun that is being translated into a match_spec. To put it simple you
cannot use anything in the fun that you cannot use in a match_spec. This means that, among others, the
following restrictions apply to the fun itself:
* Functions written in Erlang cannot be called, neither local functions, global functions or real fun's
* Everything that is written as a function call will be translated into a match_spec call to a builtin
function, so that the call is_list(X) will be translated to {'is_list', '$1'} ('$1' is just an
example, the numbering may vary). If one tries to call a function that is not a match_spec builtin,
it will cause an error.
* Variables occurring in the head of the fun will be replaced by match_spec variables in the order of
occurrence, so that the fragment fun({A,B,C}) will be replaced by {'$1', '$2', '$3'} etc. Every
occurrence of such a variable later in the match_spec will be replaced by a match_spec variable in
the same way, so that the fun fun({A,B}) when is_atom(A) -> B end will be translated into
[{{'$1','$2'},[{is_atom,'$1'}],['$2']}].
* Variables that are not appearing in the head are imported from the environment and made into
match_spec const expressions. Example from the shell:
1> X = 25.
25
2> ets:fun2ms(fun({A,B}) when A > X -> B end).
[{{'$1','$2'},[{'>','$1',{const,25}}],['$2']}]
* Matching with = cannot be used in the body. It can only be used on the top level in the head of the
fun. Example from the shell again:
1> ets:fun2ms(fun({A,[B|C]} = D) when A > B -> D end).
[{{'$1',['$2'|'$3']},[{'>','$1','$2'}],['$_']}]
2> ets:fun2ms(fun({A,[B|C]=D}) when A > B -> D end).
Error: fun with head matching ('=' in head) cannot be translated into
match_spec
{error,transform_error}
3> ets:fun2ms(fun({A,[B|C]}) when A > B -> D = [B|C], D end).
Error: fun with body matching ('=' in body) is illegal as match_spec
{error,transform_error}
All variables are bound in the head of a match_spec, so the translator can not allow multiple
bindings. The special case when matching is done on the top level makes the variable bind to '$_' in
the resulting match_spec, it is to allow a more natural access to the whole matched object. The
pseudo function object() could be used instead, see below. The following expressions are translated
equally:
ets:fun2ms(fun({a,_} = A) -> A end).
ets:fun2ms(fun({a,_}) -> object() end).
* The special match_spec variables '$_' and '$*' can be accessed through the pseudo functions object()
(for '$_') and bindings() (for '$*'). as an example, one could translate the following
ets:match_object/2 call to a ets:select call:
ets:match_object(Table, {'$1',test,'$2'}).
...is the same as...
ets:select(Table, ets:fun2ms(fun({A,test,B}) -> object() end)).
(This was just an example, in this simple case the former expression is probably preferable in terms
of readability). The ets:select/2 call will conceptually look like this in the resulting code:
ets:select(Table, [{{'$1',test,'$2'},[],['$_']}]).
Matching on the top level of the fun head might feel like a more natural way to access '$_', see
above.
* Term constructions/literals are translated as much as is needed to get them into valid match_specs,
so that tuples are made into match_spec tuple constructions (a one element tuple containing the
tuple) and constant expressions are used when importing variables from the environment. Records are
also translated into plain tuple constructions, calls to element etc. The guard test is_record/2 is
translated into match_spec code using the three parameter version that's built into match_specs, so
that is_record(A,t) is translated into {is_record,'$1',t,5} given that the record size of record type
t is 5.
* Language constructions like case, if, catch etc that are not present in match_specs are not allowed.
* If the header file ms_transform.hrl is not included, the fun won't be translated, which may result in
a runtime error (depending on if the fun is valid in a pure Erlang context). Be absolutely sure that
the header is included when using ets and dbg:fun2ms/1 in compiled code.
* If the pseudo function triggering the translation is ets:fun2ms/1, the fun's head must contain a
single variable or a single tuple. If the pseudo function is dbg:fun2ms/1 the fun's head must contain
a single variable or a single list.
The translation from fun's to match_specs is done at compile time, so runtime performance is not affected
by using these pseudo functions. The compile time might be somewhat longer though.
For more information about match_specs, please read about them in ERTS users guide.
EXPORTS
parse_transform(Forms, Options) -> Forms
Types:
Forms = [erl_parse:abstract_form()]
Options = term()
Option list, required but not used.
Implements the actual transformation at compile time. This function is called by the compiler to
do the source code transformation if and when the ms_transform.hrl header file is included in your
source code. See the ets and dbg:fun2ms/1 function manual pages for documentation on how to use
this parse_transform, see the match_spec chapter in ERTS users guide for a description of match
specifications.
transform_from_shell(Dialect, Clauses, BoundEnvironment) -> term()
Types:
Dialect = ets | dbg
Clauses = [erl_parse:abstract_clause()]
BoundEnvironment = erl_eval:binding_struct()
List of variable bindings in the shell environment.
Implements the actual transformation when the fun2ms functions are called from the shell. In this
case the abstract form is for one single fun (parsed by the Erlang shell), and all imported
variables should be in the key-value list passed as BoundEnvironment. The result is a term,
normalized, i.e. not in abstract format.
format_error(Error) -> Chars
Types:
Error = {error, module(), term()}
Chars = io_lib:chars()
Takes an error code returned by one of the other functions in the module and creates a textual
description of the error. Fairly uninteresting function actually.