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It is already allowed in match-specs.
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This complements the `map_get/2` guard BIF introduced in #1784.
Rationale.
`map_get/2` allows accessing map fields in guards, but it might be
problematic in more complex guard expressions, for example:
foo(X) when map_get(a, X) =:= 1 or is_list(X) -> ...
The `is_list/1` part of the guard could never succeed since the
`map_get/2` guard would fail the whole guard expression. In this
situation, this could be solved by using `;` instead of `or` to separate
the guards, but it is not possible in every case.
To solve this situation, this PR proposes a `is_map_key/2` guard that
allows to check if a map has key inside a guard before trying to access
that key. When combined with `is_map/1` this allows to construct a
purely boolean guard expression testing a value of a key in a map.
Implementation.
Given the use case motivating the introduction of this function, the PR
contains compiler optimisations that produce optimial code for the
following guard expression:
foo(X) when is_map(X) and is_map_key(a, X) and map_get(a, X) =:= 1 -> ok;
foo(_) -> error.
Given all three tests share the failure label, the `is_map_key/2` and
`is_map/2` tests are optimised away.
As with `map_get/2` the `is_map_key/2` BIF is allowed in match specs.
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* 'map-get-bif' of git://github.com/michalmuskala/otp:
Introduce map_get guard-safe function
OTP-15037
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Rationale
Today all compound data types except for maps can be deconstructed in guards.
For tuples we have `element/2` and for lists `hd/1` and `tl/1`. Maps are
completely opaque to guards. This means matching on maps can't be
abstracted into macros, which is often done with repetitive guards. It
also means that maps have to be always selected whole from ETS tables,
even when only one field would be enough, which creates a potential
efficiency issue.
This PR introduces an `erlang:map_get/2` guard-safe function that allows
extracting a map field in guard. An alternative to this function would be
to introduce the syntax for extracting a value from a map that was planned
in the original EEP: `Map#{Key}`.
Even outside of guards, since this function is a guard-BIF it is more
efficient than using `maps:get/2` (since it does not need to set up the
stack), and more convenient from pattern matching on the map (compare:
`#{key := Value} = Map, Value` to `map_get(key, Map)`).
Performance considerations
A common concern against adding this function is the notion that "guards
have to be fast" and ideally execute in constant time. While there are
some counterexamples (`length/1`), what is more important is the fact
that adding those functions does not change in any way the time
complexity of pattern matching - it's already possible to match on map
fields today directly in patterns - adding this ability to guards will
niether slow down or speed up the execution, it will only make certain
programs more convenient to write.
This first version is very naive and does not perform any optimizations.
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christhekeele/remove-trace-terms-from-table-types-in-match-docs
Remove trace-specific terms from docs for table-oriented matchspecs
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This removes the matchspec instructions `is_seq_trace` and `get_tcw/0`
from the documentation for table-oriented matchspecs.
This is likely correct as both are already documented under "Functions
Allowed Only for Tracing", despite appearing in the list of possible
options for table specs.
The following observations further back this change up:
```erl
erlang:match_spec_test([whatever], [{'_', [], [{is_seq_trace}]}], trace).
%=> {ok,true,[],[]}
erlang:match_spec_test({whatever}, [{'_', [], [{is_seq_trace}]}], table).
%=> {error,[{error,"Special form 'is_seq_trace' used in wrong %dialect."}]}
erlang:match_spec_test([whatever], [{'_', [], [{get_tcw}]}], trace).
%=> {ok,true,[],[]}
erlang:match_spec_test({whatever}, [{'_', [], [{get_tcw}]}], table).
%=> {error,[{error,"Function get_tcw/0 cannot be called in this context."}]}
```
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I was running this code in shell and expected it to take default `self()` meta tracer,
and start tracing `erlang:put/2`.
```
F = fun F() -> timer:sleep(5000), erlang:get(unique_field), erlang:put(unique_field, 123), F() end.
erlang:trace_pattern({erlang, put, 2}, [{['$1', '_'], [{'==', '$1', unique_field}], []}], [global]).
erlang:trace_pattern({erlang, get, 1}, [{['_'], [], [{trace, [], [call, timestamp]}]}], [meta]).
Pid = spawn(F).
```
But tracing didn't start:
```
6> flush().
Shell got {trace_ts,<0.70.0>,call,
{erlang,get,[unique_field]},
{1521,118606,753838}}
Shell got {trace_ts,<0.70.0>,call,
{erlang,get,[unique_field]},
{1521,118611,754798}}
Shell got {trace_ts,<0.70.0>,call,
{erlang,get,[unique_field]},
{1521,118616,755705}}
ok
7> erlang:trace_info(Pid, flags).
{flags,[]}
```
Turns out that I had false expectations, that `{trace, _, _}` would enable tracing on that process, inheriting meta tracer process as consumer of trace messages.
Instead it tried to take tracer from executing process (which is pointed out in docs). But there was none, so no tracing was started and flags were simply ignored (which is not that obvious from docs).
Updated docs to point out that there are cases when flags would be simply ignored, and no tracing would start.
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Fix some older errors as well.
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Introduce section/terminology "Match target".
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Tracing and ETS examples were not separated correctly
under the corresponding headings.
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Add the possibility to use modules as trace data receivers. The functions
in the module have to be nifs as otherwise complex trace probes will be
very hard to handle (complex means trace probes for ports for example).
This commit changes the way that the ptab->tracer field works from always
being an immediate, to now be NIL if no tracer is present or else be
the tuple {TracerModule, TracerState} where TracerModule is an atom that
is later used to lookup the appropriate tracer callbacks to call and
TracerState is just passed to the tracer callback. The default process and
port tracers have been rewritten to use the new API.
This commit also changes the order which trace messages are delivered to the
potential tracer process. Any enif_send done in a tracer module may be delayed
indefinitely because of lock order issues. If a message is delayed any other
trace message send from that process is also delayed so that order is preserved
for each traced entity. This means that for some trace events (i.e. send/receive)
the events may come in an unintuitive order (receive before send) to the
trace receiver. Timestamps are taken when the trace message is generated so
trace messages from differented processes may arrive with the timestamp
out of order.
Both the erlang:trace and seq_trace:set_system_tracer accept the new tracer
module tracers and also the backwards compatible arguments.
OTP-10267
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is_constant/1 was removed in R13B.
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Reported by Uwe Dauernheim.
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