How to fix bad box overfull \hbox after changing fonts?

Question

I'm trying to create an environment that takes lets me change fonts to represent an algorithm and I'm getting a Bad Box overfull \hbox error.

MWE:

\documentclass{article}
\usepackage{enumerate, lipsum}
\newenvironment{aparts}{\begin{enumerate}[{(}a{)}]}{\end{enumerate}}
\newenvironment{algo}[3] % function name and parameters, input, output
{
\fontfamily{cmtt}\selectfont
\underline{#1}
Input: #2
Output: #3
\bigskip
}
{\bigskip}
\textheight=9in
\textwidth=6.5in
\topmargin=-.75in
\oddsidemargin=0.25in
\evensidemargin=0.25in
\begin{document}
\begin{aparts}
\item
{\fontfamily{cmtt}\selectfont
\underline{function eval(a[$a_0, a_1, ..., a_n$], x)}
Input: An array of constant coefficients a[$a_0, a_1, ..., a_n$] and a quantity to plug into the polynomial expression determined by the coefficients x
Output: The solution to the polynomial expression determined by the coefficients when x is plugged in as the variable
\bigskip
$p = a_0$
if $i >= 1$:
\quad for $i = n$ to 1:

    \qquad $p = p + a_i$

     \qquad $p = p * x$


return $p$
}
\bigskip
\item
{\fontfamily{cmtt}\selectfont
\underline{function eval(a[$a_0, a_1, ..., a_n$], x)}
Input: An array of constant coefficients a[$a_0, a_1, ..., a_n$] and a quantity to
\quad plug into the polynomial expression determined by the coefficients x
Output: The solution to the polynomial expression determined by the
\quad coefficients when x is plugged in as the variable
\bigskip
$p = a_0$
if $i >= 1$:
\quad for $i = n$ to 1:

    \qquad $p = p + a_i$

    \qquad $p = p * x$


return $p$
}
\bigskip
\item
\begin{algo}{function eval(a[$a_0, a_1, ..., a_n$], x)}
{An array of constant coefficients a[$a_0, a_1, ..., a_n$] and a quantity to plug into the polynomial expression determined by the coefficients x}
{The solution to the polynomial expression determined by the coefficients when x is plugged in as the variable}
$p = a_0$
if $i >= 1$:
\quad for $i = n$ to 1:

    \qquad $p = p + a_i$

     \qquad $p = p * x$


return $p$
\end{algo}
\item
\lipsum[50]
\end{aparts}
\end{document}

I'm getting the following errors:

Overfull \hbox (4.20915pt too wide) in paragraph at lines 35--36 []\OT1/cmtt/m/n/10 Input: An array of constant coefficients a[$\OML/cmm/m/it/10 a[]; a[]; :::; a[]$\OT1/cmtt/m/n/10 ] and a quantity to plug into the []

Overfull \hbox (10.0188pt too wide) in paragraph at lines 37--38 []\OT1/cmtt/m/n/10 Output: The solution to the polynomial expression determined by the coefficients when x []

Overfull \hbox (4.20915pt too wide) in paragraph at lines 88--88 []\OT1/cmtt/m/n/10 Input: An array of constant coefficients a[$\OML/cmm/m/it/10 a[]; a[]; :::; a[]$\OT1/cmtt/m/n/10 ] and a quantity to plug into the []

Overfull \hbox (10.0188pt too wide) in paragraph at lines 88--88 []\OT1/cmtt/m/n/10 Output: The solution to the polynomial expression determined by the coefficients when x []

As you can see, the first one doesn't work, but I fixed it by brute-forcing a line break. Of course, I don't want to have to do that every time, and it's even more annoying if I'm putting it in the parameter for an environment. Also, the reason I think it has to do with fonts is that regular text looks find, as demonstrated in (d).

How can I fix this? Thanks so much.