pattern_matching_warnings

gistfile1.ex

def passwords_match?(_x, _x), do: true
def passwords_match?(_, _),   do: false

# causes the following warning...
# registration.ex:61: warning: the underscored variable "_x" appears more
# than once in a match. This means the pattern will only match
# if all "_x" bind to the same value. If this is the intended behaviour,
# please remove the leading underscore from the variable name,
# otherwise give the variables different names


# versus

def passwords_match?(x, x), do: true
def passwords_match?(_, _), do: false

# this issues no warning about unused variables even though x is not is
# matched/bound, not "ignored" (read: underscored), and not used.

# Why does the second example with `x` not cause warnings?