user_default.erl
you can build your own shell builtins by having a compiled user_default.beam in your path which can be pretty nifty
注释:这个我们在[Erlang 0027] Using Record in Erlang Shell 里面用过
Ports, external or linked-in, accept something called io-lists for sending data to them. An io-list is a binary or a (possibly deep) list of binaries or integers in the range 0..255.
This means that rather than concatenating two lists before sending them to a port, one can just send them as two items in a list. So instead of
"foo" ++ "bar"
one do
["foo", "bar"]
In this example it is of course of miniscule difference. But the iolist in itself allows for convenient programming when creating output data. io_lib:format/2,3 itself returns an io list for example.
The function erlang:list_to_binary/1 accepts io lists, but now we have erlang:iolist_to_binary/1 which convey the intention better. There is also an erlang:iolist_size/1.
Best of all, since files and sockets are implemented as ports, you can send iolists to them. No need to flatten or append.注释:这个我们专门分析过iolist [Erlang 0034] Erlang iolist
beam_lib:chunks
can get source code from a beam that was compiled with debug on which can be really usefull
{ok,{_,[{abstract_code,{_,AC}}]}} = beam_lib:chunks(Beam,[abstract_code]). io:fwrite("~s~n", [erl_prettypr:format(erl_syntax:form_list(AC))]).
注释:[Erlang 0011] Erlang 杂记Ⅱ 编号14
That match specifications
can be built using ets:fun2ms(...) where the Erlang fun syntax is used and translated into a match specification with a parse transform.
1> ets:fun2ms(fun({Foo, _, Bar}) when Foo > 0 -> {Foo, Bar} end).
[{{'$1','_','$2'},[{'>','$1',0}],[{{'$1','$2'}}]}]
So no fun-value is ever built, the expression gets replaced with the match-spec at compile-time. The fun may only do things a match expression could do.
Also, ets:fun2ms is available for usage in the shell, so fun-expressions can be tested easily.
注释: [Erlang 0008] Erlang的Match specifications
.erlang_hosts
gives a nice way to share names across machines
{packet, Type}
The gen_tcp and ssl sockets have a {packet, Type} socket option to aid in decoding a number of protocols. The function erlang:decode_packet/3 has a good description on what the various Type values can be and what they do.
Together with a {active, once} or {active, true} setting, each framed value will be delivered as a single message.
Examples: the packet http mode is used heavily for iserve and the packet fcgi mode for ifastcgi. I can imagine that many of the other http servers use packet http as well.
注释:这个绝对不是Hidden Feature,: )
.erlang
can preload libraries and run commands on a shells startup, you can also do specific commands for specific nodes by doing a case statement on node name.
注释:[Erlang 0027] Using Record in Erlang Shell
QLC
It is possible to define your own iterator for QLC to use. For example, a result set from an SQL query could be made into a QLC table, and thus benefit from the features of QLC queries.
Besides mnesia tables, dets and ets have the table/1,2 functions to return such a "Query Handle" for them.
If you want to execute more than one expression in a list comprehension, you can use a block. For example:
> [begin erlang:display(N), N*10 end || N <- lists:seq(1,3)].123[10,20,30]
hide an Erlang node
You can hide an Erlang node by starting it with:
erl -sname foo -hidden
You can still connect to the node, but it won't appear in the list returned by nodes/0.
Hot code loading.
Code is loaded and managed as "module" units, the module is a compilation unit. The system can keep two versions of a module in memory at the same time, and processes can concurrently run code from each.
The versions are referred to the "new" and the "old" version. A process will not move into the new version until it makes an external call to its module.
