P1700 - Head Mismatch

Error

    foo(0) -> 1;
    boo(1) -> 2.
 %% ^^^ error: head mismatch 'boo' vs 'foo'

Explanation

The error message indicates that two function clauses belonging the same function differ in their name or in the number of arguments.

In Erlang functions are uniquely identified by the module they belong to, the function name and the number of argument they take (known as arity). Each function can be composed by multiple clauses, separated by a semicolon (;). Therefore, all clauses belonging to the same function have to share the same name.

To fix the error you need to ensure that every function clause has the same name and that it takes the same number of arguments.

In the above example, boo/1 could be a second clause for the foo/1 function, containing a typo. In that case, the corrective action would be to fix the typo:

    foo(0) -> 1;
    foo(1) -> 2.

It could also be that boo/1 is intended to be a completely different function. In that case the error can be fixed by replacing the semicolon on the previous line with a fullstop. Leaving an empty line between the two functions would also be a good idea, to help the reader understanding foo/1 and boo/1 are two distinct functions:

    foo(0) -> 1.

    boo(1) -> 2.

For more information about Erlang functions please refer to the official documentation.