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search erlang & efene code by pattern matching code with "holes"

Why and what?

One day, for weird reasons I was looking for a data structure that would allow me to pattern match it quick without being confused by existing data, I thought "nobody would have a tuple inside a one item tuple!", but I wasn't sure.

This wasn't the first time I wanted to search code by structure, not by text.

What I mean by "by structure"?

You may want to search for stuff like:

  • Calls to function f1 on module m1 with 3 arguments
    • Same but where some argument has a specific value
  • Tuples with a specific number of items where the first item is a given atom

I guess you get the idea, you want to match some expression's structure, but not the text, because of formatting, new lines, spacing and also because some of the values are not important, you would like to match any expression in some places.

Given that I have experience generating Erlang AST (abstract syntax tree) and walking AST trees (for parse transforms and some macro magic in efene)

I decided to give it a try, and the result worked, a year later I decided to write a blog post about it :)

erlplorer search "{{_@0, _@1, _@2}}" **/src/*.erl
asn1/src/asn1ct.erl:1366 {{decode,{Module,Type,Value},Error}}
dialyzer/src/dialyzer_plt.erl:675 {{M,_F,_A}}
kernel/src/pg2.erl:300 {{local_member,Name,Pid}}
kernel/src/pg2.erl:302 {{pid,Pid,Name}}
kernel/src/pg2.erl:343 {{local_member,Name,'$1'}}
kernel/src/pg2.erl:354 {{pid,Pid,'$1'}}
mnesia/src/mnesia_locker.erl:263 {{Tid,Oid,Op}}
mnesia/src/mnesia_locker.erl:266 {{Tid,Oid,read}}
mnesia/src/mnesia_locker.erl:269 {{Tid,Oid,write}}
mnesia/src/mnesia_locker.erl:309 {{Tid,Oid,{queued,Op}}}
mnesia/src/mnesia_locker.erl:508 {{Tid,Oid,{queued,Op}}}
mnesia/src/mnesia_locker.erl:524 {{'$1','_','_'}}
mnesia/src/mnesia_locker.erl:536 {{Tid,'_','_'}}
parsetools/src/yecc.erl:1075 {{From,Sym,Next}}
parsetools/src/yecc.erl:1164 {{From,Sym,To}}
reltool/src/reltool_server.erl:764 {{'$1','$2','$3'}}

Welp, yes, there are.

But what was that?

erlplorer is a command line tool built in efene that allows to search for Erlang and efene code by providing expressions with holes.

The first argument "{{_@0, _@1, _@2}}" is an Erlang expression, in this case a one item tuple holding a 3 item tuple, the weird looking variables are a specially named variables that erlplorer interprets as "match any AST node here, I don't care", they start with _@, any other variable will match that variable name in the code.

We can see that by searching for places that match a 3 item tuple ignoring the 3 places:

erlplorer search "{_, _, _}" **/src/*.erl
asn1/src/asn1ct_check.erl:917 {_,_,_}
... to many results to show

We can use this "meta variables" to do more pattern matching, let's search for 3 item tuples that have the same thing in the 3 places:

erlplorer search "{_@, _@, _@}" **/src/*.erl
asn1/src/asn1ct_check.erl:917 {_,_,_}
asn1/src/asn1ct_constructed_ber_bin_v2.erl:701 {[],[],[]}
asn1/src/asn1ct_constructed_per.erl:401 {false,false,false}
common_test/src/ct_framework.erl:1130 {_,_,_}
common_test/src/ct_logs.erl:1492 {"","",""}

Or search for identity functions:

erlplorer search "fun(_@) -> _@ end" **/src/*.erl
asn1/src/asn1ct.erl:2099 fun(D) -> D end
asn1/src/asn1ct_gen_ber_bin_v2.erl:650 fun(V) -> V end
common_test/src/test_server.erl:2067 fun(T) -> T end
compiler/src/beam_a.erl:147 fun(L) -> L end
compiler/src/beam_jump.erl:289 fun(Old) -> Old end

Places that add 0 to something:

erlplorer search "_@ + 0" **/src/*.erl
asn1/src/asn1rtt_per_common.erl:187 N + 0
asn1/src/asn1rtt_per_common.erl:197 N + 0
stdlib/src/ms_transform.erl:90 16 + 0
stdlib/src/ms_transform.erl:92 17 + 0
stdlib/src/ms_transform.erl:97 22 + 0
stdlib/src/ms_transform.erl:102 18 + 0
stdlib/src/ms_transform.erl:106 23 + 0
stdlib/src/ms_transform.erl:111 24 + 0
stdlib/src/ms_transform.erl:167 20 + 0
stdlib/src/ms_transform.erl:170 19 + 0
stdlib/src/ms_transform.erl:174 21 + 0

Places that add the same thing:

erlplorer search "_@ + _@" **/src/*.erl
dialyzer/test/small_SUITE_data/src/maps_redef2.erl:18 A + A
stdlib/src/dets_utils.erl:1138 1 + 1
stdlib/src/dets_utils.erl:1226 1 + 1
stdlib/src/rand.erl:1464 Y + Y
stdlib/src/zip.erl:1315 Sz + Sz

You get the idea...

How to use it

You need Erlang and rebar3 installed and in your $PATH

git clone
cd erlplorer
rebar3 escriptize
# ~/bin or any other folder in your $PATH
cp _build/default/bin/erlplorer ~/bin

How is it implemented?

Wrap the expression passed in a function [1]

Compile the module to Erlang AST [2]

Extract the AST body of the dummy function [3]

"abstract the AST", that is, I take an AST as Erlang data and I generate an AST that when compiled will generate that Erlang AST, I need that because I will put that AST in a pattern match position to pattern match AST nodes as I walk Erlang ASTs [4]. yeah, meta and too many AST references

An example is worth many of my words:

% the 42 is a fake "line" where the code was supposedly parsed
1> erl_parse:abstract({foo, 100, "hi"}, 42).

We have to take care of two things:

  • vars must match an AST node for a var with that name, not act as vars that will be bound on first match and pattern matched on successive matches
  • vars that start with _@ will be compiled to actual vars that behave as vars in a pattern match, that's how we can use them to pattern match

Then compile the abstracted AST into a module with a function we can pass to ast_walk/3 [5]

Load the compiled module [6]

Parse the files passed as last argument [7]

And walk the parsed AST with our compiled matcher [8]

For each match, since we have the AST, try to pretty print it [9]

Give it a try and let me know what you think.

PS: the code is a hack I did to use it when I needed it, don't judge efene by the code you see on that project :P



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