Erlang – checking for unbound variables-Collection of common programming errors

If you (try to) refer to an unbound variable in an expression, it’s a compilation error. In particular, after

{ok, After} = ts_dynvars:lookup(last, DynVars),

there are only two possibilities: if the pattern matches, After is bound, and can be used; if it doesn’t, an exception will be thrown, and code which tries to work with After will never be executed.

UPDATE:

are you telling me there is no way to branch code execution in the situation in which the pattern does not match

Of course there is:

case ts_dynvars:lookup(last, DynVars) of
  {ok, After} -> ...;
  _ -> ... %% or other patterns
end

but the compiler won’t let you use After in other branches or after case (unless all branches bind After).

is this exception not catchable at all

It is:

try
  {ok, After} = ts_dynvars:lookup(last, DynVars),
  ...
catch
  _:_ -> ...
end

but again, you won’t be able to use After in catch sections or after try ends (you can bind a new variable named After, of course).

Originally posted 2014-01-12 20:47:49.