How to case switch in erlang using a function value? - erlang

Question

This program is crashing despite seeming to work. I don't understand why though. I'm trying to accomplish a deep merge and need conditional logic.
Given the following list:
ManOne = #{ "Bob" => #{"Sagget" => #{}} }
ManTwo = #{ "Bob" => #{"Daniels" => #{}} }
I'm trying to compare them as follows, this function returns true as expected:
check_if_same(M1, M2) ->
{ok, lists:sort( maps:keys(M1) ) == lists:sort( maps:keys(M2) )}.
merger(M1, M2) ->
M1_Keys = maps:keys(M1),
M2_Keys = maps:keys(M2),
do_merge(M1, M2, M1_Keys).
do_merge(M1, M2, [Head|Tail]) ->
Check = check_if_same(M1, M2),
io:fwrite("Check is: ~p\n", [Check]),
case Check of
{ok, true} ->
io:fwrite("true\n");
{ok, false} ->
io:fwrite("false\n")
end,
do_merge(M1, M2, Tail);
do_merge(M1, M2, []) ->
ok.
check_if_same(M1, M2) ->
{ok, lists:sort( maps:keys(M1) ) == lists:sort( maps:keys(M2) )}.
gives the following output:
Check is: {ok,true}
true
{"init terminating in do_boot",{{badmap,ok},[{maps,keys,[ok],[]},{helloworld,merger,2,[{file,"helloworld.erl"},{line,9}]},{init,start_em,1,[]},{init,do_boot,3,[]}]}}
init terminating in do_boot ()
Crash dump is being written to: erl_crash.dump...done

Answer 1

Let's go over your question first because there's a few misconceptions and/or corrections that we can make.
Your Description
This program is crashing despite seeming to work. I don't understand why though. I'm trying to accomplish a deep merge and need conditional logic.
Given the following list:
ManOne = #{ "Bob" => #{"Sagget" => #{}} }
ManTwo = #{ "Bob" => #{"Daniels" => #{}} }
Note that the above are NOT lists, they are maps, which function entirely differently.
A map is, for all intents and purposes, a lookup table until it contains ~31 key/value pairs.
At this point, it becomes a HashMap (this can be seen by viewing the elements as they become unordered after the map becomes a HashMap).
I'm trying to compare them as follows, this function returns true as expected:
check_if_same(M1, M2) ->
{ok, lists:sort( maps:keys(M1) ) == lists:sort( maps:keys(M2) )}.
This is an incorrect way to assert equality; in erlang, it is suggested to not use == to check equality.
Instead, =:= should be used.
The reason for this is due to the fact that == does NOT check the type of the elements it is comparing and only takes a fuzzy value - i.e 1 == 1.0 will return true but 1 =:= 1.0 will return false.
Personally, I would recommend instead using Erlang's Pattern-Matching to check your values.
This could be implemented using the following snippet:
-spec check_if_same(M1 :: map(), M2 :: map()) -> boolean().
check_if_same(M1, M2) ->
SortedKeys1 = lists:sort(maps:keys(M1)),
SortedKeys2 = lists:sort(maps:keys(M2)),
%% We hide the implementation of the function in
%% a function with the same name suffixed with an
%% underscore. This allows us to have a public api
%% but keep the implementation internal which allows
%% the code to be a bit cleaner.
check_if_same_(SortedKeys1, SortedKeys2).
%% If they're both empty then we've gone through
%% every key meaning that they must be identical
check_if_same_([], []) ->
true;
%% If the current Key on both heads is the same
%% then recurse and check the next and so on
check_if_same_([Key|Tail1], [Key|Tail2]) ->
check_if_same_(Tail1, Tail2);
%% If we get anything else, e.g more keys in
%% one than the other or the keys don't match,
%% then we'll fall in to this case.
%% As we know anything that falls in to this
%% case doesn't match, we just return false
check_if_same_(Keys1, Keys2) when is_list(Keys1), is_list(Keys2) ->
false.
Note that in the above snippet, I only ever returned true or false - my recommendation for cleaner code would be to keep to the following formats;
ok - This is typically for functions where you care about the effect and not the return
true | false - This is typically for comparison functions, i.e is_binary/1, is_function/1
{ok, Value} - This would typically be for any function where you care about the value returned
{error, Reason} - This would be used whenever you expect an error so that you can bubble the error back up the chain with an easy-to-match format
Your Code Snippet
merger(M1, M2) ->
M1_Keys = maps:keys(M1),
%% Note that you don't use the M2Keys here so you don't need to do the work to get them
M2_Keys = maps:keys(M2),
do_merge(M1, M2, M1_Keys).
do_merge(M1, M2, [Head|Tail]) ->
Check = check_if_same(M1, M2),
%% It's generally recommended to stick to io:format/2 rather than io:fwrite/2
io:fwrite("Check is: ~p\n", [Check]),
case Check of
{ok, true} ->
io:fwrite("true\n");
{ok, false} ->
io:fwrite("false\n")
end,
do_merge(M1, M2, Tail);
do_merge(M1, M2, []) ->
ok.
check_if_same(M1, M2) ->
{ok, lists:sort( maps:keys(M1) ) == lists:sort( maps:keys(M2) )}.
Now, the above snippet (other than being a bit inefficient) is perfectly okay erlang and will work as expected
gives the following output:
Check is: {ok,true}
true
{"init terminating in do_boot",{{badmap,ok},[{maps,keys,[ok],[]},{helloworld,merger,2,[{file,"helloworld.erl"},{line,9}]},{init,start_em,1,[]},{init,do_boot,3,[]}]}}
init terminating in do_boot ()
Crash dump is being written to: erl_crash.dump...done
This crash dump is where the real issue is;
Check is: {ok,true}
true
From this we can tell that we
Hit the io:fwrite/2 (io:fwrite("Check is: ~p\n", [Check]))
Entered the {ok, true} path in the case (io:fwrite("true\n"))
The next line is where we see the actual problem, let's break it down:
"init terminating in do_boot" - We failed when starting up, this might be when running an escript or starting an app
Now let's break down that tuple:
{
{badmap,ok}, %% The function we called expected a map and we passed in 'ok'
[
{maps,keys,[ok],[]}, %% We called maps:keys/1 with 'ok' as an arg
{helloworld,merger,2,[{file,"helloworld.erl"},{line,9}]}, %% This was called at helloworld:merger/2 (helloworld.erl:9)
{init,start_em,1,[]},{init,do_boot,3,[]} %% We failed on start up
]
}
What we can take away from this is that you're calling merger in your code with an invalid value of ok on line 9 of helloworld.erl

Answer 2

There is some missing information. Although this code looks like a first draft or step, it works as expected. I tested it in the shell and got this:
-module (merger).
-compile(export_all).
merger(M1, M2) ->
M1_Keys = maps:keys(M1),
M2_Keys = maps:keys(M2),
do_merge(M1, M2, M1_Keys).
do_merge(M1, M2, [Head|Tail]) ->
Check = check_if_same(M1, M2),
io:fwrite("Check is: ~p\n", [Check]),
case Check of
{ok, true} ->
io:fwrite("true\n");
{ok, false} ->
io:fwrite("false\n")
end,
do_merge(M1, M2, Tail);
do_merge(M1, M2, []) ->
ok.
check_if_same(M1, M2) ->
{ok, lists:sort( maps:keys(M1) ) == lists:sort( maps:keys(M2) )}.
test() ->
merger(#{ "Bob" => #{"Sagget" => #{}} },#{ "Bob" => #{"Daniels" => #{}} }).
which gives:
8> c(merger).
merger.erl:3: Warning: export_all flag enabled - all functions will be exported
merger.erl:7: Warning: variable 'M2_Keys' is unused
merger.erl:9: Warning: variable 'Head' is unused
merger.erl:19: Warning: variable 'M1' is unused
merger.erl:19: Warning: variable 'M2' is unused
{ok,merger}
9> merger:test().
Check is: {ok,true}
true
ok
10>
Maybe you could also tell us what is the expected result of merging ManOne and ManTwo