CODEWITHSUNDEEP
erlangpattern-matching
farhanhubble
farhanhubble

Reputation: 494

Erlang Doesn't Warn About Unused Function Argument

If I declare a function

test(A) -> 3.

Erlang generates a warning about variable A not being used. However the definition

isEqual(X,X) -> 1.

Doesn't produce any warning but 

isEqual(X,X) -> 1;
isEqual(X,Y) -> 0.

again produces a warning but only for the second line.

Upvotes: 1

Views: 175

Answers (1)

juan.facorro
juan.facorro

Reputation: 9930

The reason why that doesn't generate a warning is because in the second case you are asserting (through pattern matching), by using the same variable name, that the first and second arguments to isEqual/2 have the same value. So you are actually using the value of the argument.

It might help to understand better if we look at the Core Erlang code produced from is_equal/2. You can get .core source files by compiling your .erl file in the following way: erlc +to_core pattern.erl (see here for pattern.erl).

This will produce a pattern.core file that will look something like this (module_info/[0,1] functions removed):

module 'pattern' ['is_equal'/2]
  attributes []

'is_equal'/2 = fun (_cor1,_cor0) ->
                   case <_cor1,_cor0> of
                     %% Line 5
                     <X,_cor4> when call 'erlang':'=:=' (_cor4, X) ->
                       1
                     %% Line 6
                     <X,Y> when 'true' ->
                       0
                   end

As you can see, each function clause from is_equal/2 in the .erl source code gets translated to a case clause in Core Erlang. X does get used in the first clause since it needs to be compared to the other argument. On the other hand neither X or Y are used in the second clause.

Upvotes: 6

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