Equality and the Erlang standard library

Erlang’s notion of equality can be confusing, especially when dealing with numeric types. Recall the Erlang term order:

number < atom < reference < fun < port < pid < tuple < list < binary

• Lists are compared element by element.
• Tuples are ordered by size, two tuples with the same size are compared element by element.

Most operators promote integer operands to floats.

|-----|----------|
| op  | 1 op 1.0 |
|-----|----------|
| ==  | true     |
| =:= | false    |
| /=  | false    |
| =/= | true     |
|-----|----------|

=< and >= use ==-equality. We have:

Eshell V5.7.4  (abort with ^G)
1> [1, 2] < [2].
true
2> {1.0} =< {1}.
true

Particular care must be taken when using numbers as part of the key in one of the standard collections. Trying to look up the result of an arithmetic expression involving a float in a collection which uses =:= internally often annoys.

|--------------|-----|
| module       | op  |
|--------------|-----|
| dict         | =:= |
| orddict      | ==  |
| proplists    | =:= |
| gb_trees     | ==  |
| sets         | =:= |
| ordsets      | ==  |
| gb_sets      | ==  |
| lists:key*   | ==  |
| lists:ukey*  | ==  |
| lists:delete | =:= |
| lists:member | =:= |
|--------------|-----|

C.f. this recent thread on the erlang-questions list.