When pattern matching maps in Erlang, why is this variable unbound?

Question

-module(count).
-export([count/1]).

count(L) when is_list(L) ->
  do_count(L, #{});
count(_) ->
  error(badarg).

do_count([], Acc) -> Acc;
do_count([H|T],         


        

Answer 1

I made up this rule for myself: when using maps in the head of a function clause, the order of matching is not guaranteed. As a result, in your example you can't count on a [H|T] match to provide a value for H.

Several features of maps look like they should work, and Joe Armstrong says they should work, but they don't. It's a dumb part of erlang. Witness my incredulity here: https://bugs.erlang.org/browse/ERL-88

Simpler examples:

do_stuff(X, [X|Y]) ->
    io:format("~w~n", [Y]).

test() ->
    do_stuff(a, [a,b,c]).

4> c(x).
{ok,x}

5> x:test().
[b,c]
ok

But:

-module(x).
-compile(export_all).

do_stuff(X, #{X := Y}) ->
    io:format("~w~n", [Y]).

test() ->
    do_stuff(a, #{a => 3}).


8> c(x).
x.erl:4: variable 'X' is unbound