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:

We create the table using:

Let's also fill it with some randomly chosen data for the examples:

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:

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:

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:

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):

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:

This gives the same result, the 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

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:

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 ,}]]]> created:

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 }]]>, for each hired 1997 or later, but before 2001, return }]]>, for all others return }]]>. 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; }]]>:

The result will be:

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:

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

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 )]]>:

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:

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:

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:

And with the same tracing turned on, we get the following trace output:

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:

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:

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:

1> X = 25.
25
2> ets:fun2ms(fun({A,B}) when A > X -> B end).
[{{'$1','$2'},[{'>','$1',{const,25}}],['$2']}]

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}        

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.