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L1327 - Match in Comprehension Qualifier

Error​

  foo(L) ->
    [X || X = hd(L)].
%%        ^ error: matches using '=' are not allowed in comprehension qualifiers

Explanation​

Using = matches in comprehension qualifiers (filters) is not allowed unless the experimental compr_assign language feature is enabled.

With compr_assign enabled, a match P = E in a comprehension qualifier behaves as a strict generator P <-:- [E].

%% Instead of:
foo(L) -> [X || X = hd(L)].

%% Write:
foo(L) ->
    X = hd(L),
    [X].

To enable the feature, add -feature(compr_assign, enable). to your module.