Preservation of empty lines is broken

[elbrujohalcon]

A file like this one…

-module(f_bench).

-compile(export_all).

a_fun(With, Some, Arguments) ->
    an:expression(that,
                  occupies,
                  Some,
                  #{lines => since, it => uses, a => [long, list], 'of' => Arguments}),


    %% a couple of empty lines below the aforementioned long expression
    another:expression(With, smaller, size),

    %% an empty line that is preserved
    ok.

…results in…

-module(f_bench).

-compile(export_all).

a_fun(With, Some, Arguments) ->
    an:expression(that,
                  occupies,
                  Some,
                  #{lines => since, it => uses, a => [long, list], 'of' => Arguments}),
    %% a couple of empty lines below the aforementioned long expression
    another:expression(With, smaller, size),

    %% an empty line that is preserved
    ok.

…while it should result in…

-module(f_bench).

-compile(export_all).

a_fun(With, Some, Arguments) ->
    an:expression(that,
                  occupies,
                  Some,
                  #{lines => since, it => uses, a => [long, list], 'of' => Arguments}),

    %% a couple of empty lines below the aforementioned long expression
    another:expression(With, smaller, size),

    %% an empty line that is preserved
    ok.

[elbrujohalcon]

The issue is in the second clause of default_formatter:is_last_and_before_empty_line/2:

is_last_and_before_empty_line(H, [], #ctxt{empty_lines = EmptyLines}) ->
    lists:member(get_pos(H) + 1, EmptyLines);
is_last_and_before_empty_line(H, [H2 | _], #ctxt{empty_lines = EmptyLines}) ->
    get_pos(H2) - get_pos(H) >= 2 andalso lists:member(get_pos(H) + 1, EmptyLines).

get_pos(H) + 1 may not be empty, because it might be part of H if H uses more than one line to be printed. Maybe we should use get_pos(H2) - 1 instead… or maybe figure out if any of the lines between get_pos(H2) and get_pos(H) is empty.

[elbrujohalcon]

This was not completely fixed, see https://github.com/AdRoll/erliam/pull/5/files#r409081479

[elbrujohalcon]

Actually… that thing is by design. The formatter does not preserve empty spacing between clauses. It doesn't preserve it between function clauses, case clauses, if clauses, etc…
Only between statements.