Why does substring not work inside equal?

Question

Why does this code output "False":
documentclass{article}
usepackage{stringstrings}
usepackage{xifthen}

begin{document}
ifthenelse{equal{substring{ab}{1}{1}}{a}}{True}{False}
end{document}

There are many alternative solutions to get the desired result: Test if the first character of a string is 'a', but I would like tho understand why the above code behaves this way and what would be the closest possible solution to get the desired behavior.

egreg · Accepted Answer

The macro substring does not work by pure expansion: it produces a (long) series of commands to print the requested substring.
With [q] you can make it save the result in thestring. Well, it always does, but [q] suppresses output at the calling spot.
documentclass{article}
usepackage{stringstrings}
usepackage{xifthen}

begin{document}
substring[q]{ab}{1}{1}
ifthenelse{equal{thestring}{a}}{True}{False}
end{document}

This will print “True”.
I'd prefer the more powerful expl3 routine:
documentclass{article}
usepackage{xparse}
usepackage{xifthen}

ExplSyntaxOn
NewExpandableDocumentCommand{substring}{mmm}
 {
  tl_range:nnn { #1 } { #2 } { #3 }
 }
ExplSyntaxOff

begin{document}

ifthenelse{equal{substring{ab}{1}{1}}{a}}{True}{False}

ifthenelse{equal{substring{abcde}{-2}{-1}}{de}}{True}{False}

end{document}

You see that you can also extract substrings from the end. See also https://tex.stackexchange.com/a/467527/4427 for a reimplementation of ifthenelse with expl3.

Mico · Answer

If you're free to use LuaLaTeX, consider using Lua's powerful string functions, including string.sub (short for "string subset", I suppose).
For instance, the following program defines a LaTeX macro called TestFirstChar. TestFirstChar{ab}{a} typesets "True".
documentclass{article}
newcommandTestFirstChar[2]{%
    directlua{if (string.sub("#1",1,1)=="#2") then tex.sprint("True")
               else tex.sprint("False") 
               end}}

begin{document}
TestFirstChar{ab}{a}
end{document}

Why does substring not work inside equal?

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