Introduce is_map_key/2 guard BIF

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.

1 files changed, 18 insertions, 0 deletions

diff --git a/lib/compiler/test/map_SUITE.erl b/lib/compiler/test/map_SUITE.erl
index e98c295da6..6badc7a8b8 100644
--- a/lib/compiler/test/map_SUITE.erl
+++ b/lib/compiler/test/map_SUITE.erl
@@ -1203,12 +1203,18 @@ t_guard_bifs(Config) when is_list(Config) ->
     true   = map_guard_empty_2(),
     true   = map_guard_head(#{a=>1}),
     false  = map_guard_head([]),
+    true   = map_get_head(#{a=>1}),
+    false  = map_get_head([]),
+    true   = map_is_key_head(#{a=>1}),
+    false  = map_is_key_head(#{}),
     true   = map_guard_body(#{a=>1}),
     false  = map_guard_body({}),
     true   = map_guard_pattern(#{a=>1, <<"hi">> => "hi" }),
     false  = map_guard_pattern("list"),
     true   = map_guard_tautology(),
     true   = map_guard_ill_map_size(),
+    true   = map_field_check_sequence(#{a=>1}),
+    false  = map_field_check_sequence(#{}),
     ok.
 
 map_guard_empty() when is_map(#{}); false -> true.
@@ -1218,6 +1224,12 @@ map_guard_empty_2() when true; #{} andalso false -> true.
 map_guard_head(M) when is_map(M) -> true;
 map_guard_head(_) -> false.
 
+map_get_head(M) when map_get(a, M) =:= 1 -> true;
+map_get_head(_) -> false.
+
+map_is_key_head(M) when is_map_key(a, M) -> true;
+map_is_key_head(M) -> false.
+
 map_guard_body(M) -> is_map(M).
 
 map_guard_pattern(#{}) -> true;
@@ -1227,6 +1239,12 @@ map_guard_tautology() when #{} =:= #{}; true -> true.
 
 map_guard_ill_map_size() when true; map_size(0) -> true.
 
+map_field_check_sequence(M)
+  when is_map(M) andalso is_map_key(a, M) andalso (map_get(a, M) == 1) ->
+    true;
+map_field_check_sequence(_) ->
+    false.
+
 t_guard_sequence(Config) when is_list(Config) ->
 	{1, "a"} = map_guard_sequence_1(#{seq=>1,val=>id("a")}),
 	{2, "b"} = map_guard_sequence_1(#{seq=>2,val=>id("b")}),