LaTeX3 - Test string equality + Star version

Question

I'm looking for some good advice on switching from (La)TeX to LaTeX3
Here is the first questions corresponding to the toy code below.

What is a good way to manage an optional argument with LaTeX3?

What is a good way to define star versions with LaTeX3?

documentclass[12pt,a4paper]{article}

usepackage{ifthen}

makeatletter

newcommandtest{@ifstar{@test@star}{@test@no@star}}

newcommand@test@no@star[1]{%
    I am a %
    ifthenelse{equal{#1}{upper}}%
               {TSAR}%
               {tsar}%
    .
}

newcommand@test@star{%
    I am a star.%
}

makeatother

begin{document}

test{}

test{upper}

test*{}

end{document}

egreg · Accepted Answer

Defining a command with a *-variant is very simple:
NewDocumentCommand{test}{sm}{%
  IfBooleanTF{#1}
    {% * is present
     I am a star.%
    }
    {% no *
     testupper{#2}%
    }%
}

How to define testupper to check whether the argument is upper? There is no “user level version”, and you need to go to the expl3 level; defining your own comparator is recommended:
ExplSyntaxOn
NewExpandableDocumentCommand{comparestringTF}{mmmm}
 {% #1 = input string, #2 = fixed string, #3 = true text, #4 = false text
  str_if_eq:nnTF { #1 } { #2 } { #3 } { #4 }
 }
ExplSyntaxOff

NewDocumentCommand{testupper}{m}{%
  I am a comparestringTF{#1}{upper}{TSAR}{tsar}%
}

In these examples you could use NewExpandableDocumentCommand everywhere, but don't be too fast in employing it.

David Carlisle · Answer

There isn't really a "LaTeX3" just an up to date LaTeX2e
documentclass{article}

NewDocumentCommandtest{s}{IfBooleanTF{#1}{star}{no star}}

begin{document}

test

test*

end{document}

Don Hosek · Answer

You'll want to use the xparse capabilities that have been part of LaTeX's kernel since last October. Your definition becomes:
NewDocumentCommand{test}{ s }{%
   I am a 
   IfBooleanTF{#1}
     {TSAR}
     {tsar}
}

The full dirt on xparse can be had by typing texdoc xparse at a command line or searching for xparse at https://texdoc.org

LaTeX3 - Test string equality + Star version

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