Problem with nested IfSubStr calls

Question

I have the following code in my preamble:

\usepackage{xstring}
\newcommand\myhelpermacro[1]{\IfSubStr{#1}{true}{Text contained the word "cat"}{Text did not contain the word "cat"}}
\newcommand\mymacro[1]{\myhelpermacro{\IfSubStr{#1}{cat}{true}{false}}}

When I later try to use mymacro in the document, like so:

\mymacro{cat}

I get the following error:

Use of \@xs@IfSubStr@@ doesn't match its definition. \mymacro{cat}

Modifying mymacro by using \expandafter, like so:

\newcommand\mymacro[1]{\expandafter\myhelpermacro{\IfSubStr{#1}{cat}{true}{false}}}

doesn't help. However, if I instead manually expand the call to IfSubStr in mymacro, i.e.

\newcommand\mymacro[1]{\myhelpermacro{true}}

the code works.

What is going on here? Why doesn't my code work without manually expanding IfSubStr?

---

Edit 1: To clarify my expectations of the behavior of the code, I expect that when I use, e.g.,

\mymacro{caterpillar}

this expands to

\myhelpermacro{\IfSubStr{caterpillar}{cat}{true}{false}}

which in turn should expand (since I expect what's inside the brackets to be expanded before what's outside of them, because that's how brackets are supposed to behave, right? :) ) to

\myhelpermacro{true}

which in turn should expand to Text contained the word "cat".

However, when I use, e.g.,

\mymacro{aardvark}

I expect this to expand to

\myhelpermacro{\IfSubStr{aardvark}{cat}{true}{false}}

which in turn should expand to

\myhelpermacro{false}

which in turn should expand to Text did not contain the word "cat".

---

Edit 2: After reading the xstring documentation (under 3.2 Expansion of macros, optional argument), I tried changing the definition of mymacro, by storing the result of IfSubStr in a temporary variable, to

\newcommand\mymacro[1]{\IfSubStr{#1}{cat}{true}{false}[\result]\myhelpermacro{\‌​result}}

only to get the following error message when I try to use mymacro:

Undefined control sequence. \mymacro{cat}

Suddenly, LaTeX seems to think mymacro isn't defined at all. Why?

---

Edit 3: So what I'm trying to do is to define two macros:

The first macro (myhelpermacro in this case) that takes a boolean value and inserts a string into the document whose value depends on the value of the boolean.

The second macro (mymacro in this case) takes a string and calls the first macro with the boolean true if the string contains a specific substring, otherwise it calls the first macro with the boolean false.

Since I want to be able to make calls to both macros from the document, both of them need to be defined, and since I want to avoid code duplication, the second macro should call the first macro instead of inserting the string into the document itself.

Answer 1

With functional package, composition of functions works as expected if you are familiar with other programming languages such as Lua or JavaScript.

\documentclass{article}
\usepackage{functional}
\begin{document}

\IgnoreSpacesOn
\PrgNewFunction \mymacro {m} {
  \myhelpermacro{\StrIfInTF{#1}{cat}{\Result{true}}{\Result{false}}}
}
\PrgNewFunction \myhelpermacro {m} {
  \StrIfInTF {#1} {true} {
    \Result{Text ~ contained ~ the ~ word ~ "cat"}
  }{
    \Result{Text ~ did ~ not ~ contain ~ the ~ word ~ "cat"}
  }
}
\IgnoreSpacesOff

\mymacro{caterpillar}

\mymacro{butterfly}

\end{document}

Note that with this package, you need to pass return values of functions with \Result command.