From 5d0874a8f9fd617308d9024783db1e4e24268184 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Muska=C5=82a?= <michal@muskala.eu>
Date: Fri, 27 Apr 2018 12:40:07 +0200
Subject: 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.
---
 lib/stdlib/src/erl_internal.erl | 2 ++
 lib/stdlib/src/ms_transform.erl | 1 +
 2 files changed, 3 insertions(+)

(limited to 'lib/stdlib')

diff --git a/lib/stdlib/src/erl_internal.erl b/lib/stdlib/src/erl_internal.erl
index 6d3d5baa23..dd509191ef 100644
--- a/lib/stdlib/src/erl_internal.erl
+++ b/lib/stdlib/src/erl_internal.erl
@@ -109,6 +109,7 @@ new_type_test(is_function, 2) -> true;
 new_type_test(is_integer, 1) -> true;
 new_type_test(is_list, 1) -> true;
 new_type_test(is_map, 1) -> true;
+new_type_test(is_map_key, 2) -> true;
 new_type_test(is_number, 1) -> true;
 new_type_test(is_pid, 1) -> true;
 new_type_test(is_port, 1) -> true;
@@ -315,6 +316,7 @@ bif(is_function, 2) -> true;
 bif(is_integer, 1) -> true;
 bif(is_list, 1) -> true;
 bif(is_map, 1) -> true;
+bif(is_map_key, 2) -> true;
 bif(is_number, 1) -> true;
 bif(is_pid, 1) -> true;
 bif(is_port, 1) -> true;
diff --git a/lib/stdlib/src/ms_transform.erl b/lib/stdlib/src/ms_transform.erl
index ec8cfd56c2..428c23524b 100644
--- a/lib/stdlib/src/ms_transform.erl
+++ b/lib/stdlib/src/ms_transform.erl
@@ -929,6 +929,7 @@ bool_test(is_port,1) -> true;
 bool_test(is_reference,1) -> true;
 bool_test(is_tuple,1) -> true;
 bool_test(is_map,1) -> true;
+bool_test(is_map_key, 2) -> true;
 bool_test(is_binary,1) -> true;
 bool_test(is_function,1) -> true;
 bool_test(is_record,2) -> true;
-- 
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