I want to create a function for use as a predicate in the function lists:any. I try it first in erlang shell,

137> F = fun(X) -> X == 1 end.
#Fun<erl_eval.6.80484245>

It works as expected:

138> lists:any(F, [2, 3]).
false
139> lists:any(F, [2, 1, 3]).
true

Then I want to change the condition to something else, and I got an error message:

140> F = fun(X) -> X /= 1 end.
** exception error: no match of right hand side value #Fun<erl_eval.6.80484245>

My first thought: It is interesting… Did I mix up the ‘not equal’ operator in erlang? Maybe it is not ‘/=’?

It turns out it is because the code at 140 is effectively an attempt to rebound the F variable. The error message is actually about this rather than the operator. I do not usually test code in the erlang shell, so it took me a while to figure it out.