`EOFError` in the REPL on pressing `ctrl+t` after typing a single character

[jishnub]

Steps to reproduce

julia> # press any alphanumeric character key here, eg 'a', followed by ctrl + t

julia> a┌ Error: Error in the keymap
│   exception =
│    EOFError: read end of file
│    Stacktrace:
│      [1] read
│        @ ./iobuffer.jl:217 [inlined]
│      [2] read
│        @ ./io.jl:776 [inlined]
│      [3] edit_transpose_chars(buf::IOBuffer)
│        @ REPL.LineEdit ~/packages/julias/julia-latest/share/julia/stdlib/v1.9/REPL/src/LineEdit.jl:1094
│      [4] edit_transpose_chars(s::REPL.LineEdit.MIState)
│        @ REPL.LineEdit ~/packages/julias/julia-latest/share/julia/stdlib/v1.9/REPL/src/LineEdit.jl:1086
│      [5] (::REPL.LineEdit.var"#163#218")(::REPL.LineEdit.MIState, ::Any, ::Vararg{Any})
│        @ REPL.LineEdit ~/packages/julias/julia-latest/share/julia/stdlib/v1.9/REPL/src/LineEdit.jl:2397
│      [6] #invokelatest#2
│        @ ./essentials.jl:799 [inlined]
│      [7] invokelatest
│        @ ./essentials.jl:796 [inlined]
│      [8] (::REPL.LineEdit.var"#27#28"{REPL.LineEdit.var"#163#218", String})(s::Any, p::Any)
│        @ REPL.LineEdit ~/packages/julias/julia-latest/share/julia/stdlib/v1.9/REPL/src/LineEdit.jl:1533
│      [9] prompt!(term::REPL.Terminals.TextTerminal, prompt::REPL.LineEdit.ModalInterface, s::REPL.LineEdit.MIState)
│        @ REPL.LineEdit ~/packages/julias/julia-latest/share/julia/stdlib/v1.9/REPL/src/LineEdit.jl:2665
│     [10] run_interface(terminal::REPL.Terminals.TextTerminal, m::REPL.LineEdit.ModalInterface, s::REPL.LineEdit.MIState)
│        @ REPL.LineEdit ~/packages/julias/julia-latest/share/julia/stdlib/v1.9/REPL/src/LineEdit.jl:2567
│     [11] run_frontend(repl::REPL.LineEditREPL, backend::REPL.REPLBackendRef)
│        @ REPL ~/packages/julias/julia-latest/share/julia/stdlib/v1.9/REPL/src/REPL.jl:1253
│     [12] (::REPL.var"#49#54"{REPL.LineEditREPL, REPL.REPLBackendRef})()
│        @ REPL ./task.jl:494
└ @ REPL.LineEdit ~/packages/julias/julia-latest/share/julia/stdlib/v1.9/REPL/src/LineEdit.jl:2667

julia> VERSION
v"1.9.0-DEV.624"

From the discourse thread, I understand that ctrl+t performs a character transpose. Perhaps this should be a no-op if there's only one character on the line?

[perryprog]

Easiest fix is to (ironically) transpose the first two lines of edit_transpose_chars(buf::IOBuffer) to be

function LineEdit.edit_transpose_chars(buf::IOBuffer)
    eof(buf) && char_move_left(buf)
    position(buf) == 0 && return false
    char_move_left(buf)
    pos = position(buf)
    a, b = read(buf, Char), read(buf, Char)
    seek(buf, pos)
    write(buf, b, a)
    return true
end

While this will move the cursor left by one even when it doesn't transpose anything, that does actually matches the behavior of Emac's ^T when used in a buffer of just a single letter and the point is at the end of the line.

Edit: giving a shot at implementing this; seems like a fun first Julia PR! :)

[Keno]

I'm still seeing this issue when hitting ctrl-t on a completely empty buffer.

[perryprog]

Whoops; sorry about that, and thanks for the catch. Should be fixed in the above.