How to use FileTemplate instead of Splice that has been superseded?

Question

Having invoked Splice[main.mtex],

% main.mtex

\documentclass[preview,border=12pt,12pt]{standalone}
\usepackage{amsmath}
% <* f[x_] := x^2 - 5 x +6 *>
\begin{document}
\section{Problem}
Evaluate $ <* f[x] *> $ for $x=\frac 1 2$.

\section{Solution}
$$ <* f[1/2] *> $$

\end{document}

we have main.tex

% main.mtex

\documentclass[preview,border=12pt,12pt]{standalone}
\usepackage{amsmath}
% \text{Null}
\begin{document}
\section{Problem}
Evaluate $ x^2-5 x+6 $ for $x=\frac 1 2$.

\section{Solution}
$$ \frac{15}{4} $$

\end{document}

that will produce the following output when compiled with pdflatex.

Question

How to use FileTemplate instead of Splice that has been superseded by FileTemplate?

Answer 1

the essential equivalent of Splice is:

 FileTemplateApply[FileTemplate["main.mtxt"], "main.txt"]

which produces:

 % main.mtex
\documentclass[preview,border=12pt,12pt]{standalone}
\usepackage{amsmath}
% Null
\begin{document}
\section{Problem}
Evaluate $ Plus[6, Times[-5, x], Power[x, 2]] $ for $x=\frac 1 2$.
\section{Solution}
$$ 3.75 $$
\end{document}

evidently its applying FullForm to the first expression and CForm to the numeric result, while Splice seems to use CForm for both.

this seems to fix the formatting:

FileTemplateApply[
 FileTemplate["main.mtxt", 
  InsertionFunction -> (ToString[InputForm[#]] &)],"main.txt"]

 % main.mtex
\documentclass[preview,border=12pt,12pt]{standalone}
\usepackage{amsmath}
% Null
\begin{document}
\section{Problem}
Evaluate $ 6 - 5*x + x^2 $ for $x=\frac 1 2$.
\section{Solution}
$$ 15/4 $$
\end{document} 

better yet..

insert[v_Rational] := 
 TemplateApply[
  StringTemplate["\\frac{`1`}{`2`}"], {Numerator[v], Denominator[v]}]
insert[v_] := ToString[TraditionalForm[v]]
FileTemplateApply[FileTemplate["main.mtxt",
    InsertionFunction :> insert],"main.txt"]

% main.mtex
\documentclass[preview,border=12pt,12pt]{standalone}
\usepackage{amsmath}
% Null
\begin{document}
\section{Problem}
Evaluate $ x^2-5 x+6 $ for $x=\frac 1 2$.
\section{Solution}
$$ \frac{15}{4} $$
\end{document} 

Be aware TraditionalForm will have undesirable results if the output wants to be multi-line, InputForm is safer