Sfrgttk

I learnt that parbox has five arguments, that is: parbox[<align>][<height>][<inner-align>]{<width>}{<text>}. So I constrcut my own myparbox in which I add sloppysetlengthparfillskip{0pt} before #5. But it fails. What's the reason and how to re-define myparbox?

MWE:

documentclass{article}

usepackage{xparse}

NewDocumentCommand{myparbox}{ooomm}{%

  parbox[#1][#2][#3]{#4}{sloppysetlengthparfillskip{0pt}#5}

}

begin{document}

AAAfbox{parbox[][][]{4em}{aa bb cc dd ee ff}}AAA\% parbox typesets nothing. why?

BBBfbox{myparbox{4em}{aa bb cc dd ee ff}}BBB% This fails to compile

end{document}

EDIT:

documentclass{article}

usepackage{xparse}

letoldparboxparbox

RenewDocumentCommand{parbox}{sO{c}oO{t}mm}{%

  IfBooleanTF{#1}

    {%

      IfNoValueTF{#3}

        {oldparbox[#2]{#5}{sloppysetlengthparfillskip{0pt}#6}}

        {oldparbox[#2][#3][#4]{#5}{sloppysetlengthparfillskip{0pt}#6}}

    }

    {%

      IfNoValueTF{#3}

        {oldparbox[#2]{#5}{#6}}

        {oldparbox[#2][#3][#4]{#5}{#6}}

    } 

}

begin{document}thefboxsep

AAAfbox{parbox[t]{8em}{aa bb cc dd ee ff gg hh ii}}AAA\% parbox typesets nothing why?.

BBBfbox{parbox*{6em}{aa bb cc dd ee ff}}BBB% This fails to compile

end{document}

As noted in the comments, an empty optional argument [] need not be equivalent to giving no optional argument at all. Whether not giving the argument is equivalent to passing a particular value must be checked with the documentation or implementation of the command.

In the example things get worse because xparse's optional o argument actually contain the special marked value -NoValue- if the corresponding optional argument was not given. You can and should test for presence of a value with IfNoValueTF (as suggested by daleif's comment).

That is a very viable solution when you have only one optional argument to deal with, but it gets messy if the number of arguments increases.

In case of parbox you can find out that the default arguments are c, relax as special marker and s when you look up the definition in sourc2e.pdf.

So you could try

documentclass{article}

usepackage{xparse}

NewDocumentCommand{myparbox}{O{c}O{relax}O{s}mm}{%

  parbox[#1][#2][#3]{#4}{sloppysetlengthparfillskip{0pt}#5}%

}

begin{document}

BBBfbox{parbox{4em}{sloppysetlengthparfillskip{0pt}aa bb cc dd ee ff}}BBB



BBBfbox{myparbox{4em}{aa bb cc dd ee ff}}BBB

end{document}

Normally I would try to work around issues like this that require me to know the default optional argument values/behaviour and or need a great number of IfNoValueTF tests to get the arguments right as the first method seems fragile and the second very verbose and repetitive. An approach similar to Marijn's answer might be an alternative, but at least in this case also requires intimate knowledge of the definition of parbox.

edit: I only just saw the new version of the question. let is not enough for robust commands with optional arguments: When to use LetLtxMacro?. I would also strongly suggest not to redefine fundamental commands such as parbox even if the implementation might be backwards compatible. A new name is much safer.

Alternatively, you can use xpatch to insert the extra code at the right place in the command, without the need to pass variables around. The parbox command calls the internal @iiiparbox command which processes the actual contents, so this internal command is the one that should be patched. MWE:

documentclass{article}

usepackage{xpatch}

begin{document}

makeatletter

xpatchcmd{@iiiparbox}{#5}{sloppysetlengthparfillskip{0pt}#5}{}{}

makeatother

AAAfbox{parbox{4em}{aa bb cc dd ee ff}}AAA

end{document}

Note that this changes the behavior of all parboxes. If you want a custom command with the justified alignment and also keep the regular command, then you can define the custom command using a copy of the @iiiparbox command and patch the copy instead of the original. MWE:

documentclass{article}

usepackage{xpatch}

begin{document}

makeatletter

defmyparbox{@ifnextchar [@iparbox {myiiiparbox crelax [s]}}

letmyiiiparbox@iiiparbox

xpatchcmd{myiiiparbox}{#5}{sloppysetlengthparfillskip{0pt}#5}{}{} 

makeatother

AAAfbox{parbox{4em}{aa bb cc dd ee ff}}AAA



BBBfbox{myparbox{4em}{aa bb cc dd ee ff}}BBB

end{document}

Result: