\documentclass[a4paper]{article}
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\usepackage{multicol}

\renewcommand{\thesection}{Task \arabic{section}}
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\title{W2 - IPS}
\author{Nikolaj Gade (qhp695)}
\date{May 2022}

\newcommand{\reg}[1]{%
\begin{center}
  \large
  \texttt{#1}
\end{center}
}

\begin{document}
\maketitle
\section{}
\subsection{}
\begin{multicols}{2}
\subsubsection{}
The string must consist of any number of pairs of '\texttt{o}'s and '\texttt{g}'s. The pairs can be any combination of 2 characters, both of which are either '\texttt{o}' or '\texttt{g}'. Thus, each character can be written as, '\texttt{o|g}', and each pair as '\texttt{(o|g)(o|g)}'. The full string can be written as:

\reg{((o|g)(o|g))*}

\subsubsection{}
The first character must be '\texttt{o}', and it must be followed by a character that is either '\texttt{o}' or '\texttt{g}'. Following that, the answer is the same as the previous problem:

\reg{o(o|g)((o|g)(o|g))*}

\columnbreak

\subsubsection{}
Keeping with the idea that an even number of characters can be split up in "pairs" of characters, an odd number of '\texttt{o}'s or '\texttt{g}'s \textit{must} result in an odd number of both. Thus, the string will either consist of an even number of '\texttt{o}'s, followed by an even number of '\texttt{g}'s, \textbf{or} an even number of '\texttt{o}'s, followed by '\texttt{og}', followed by an even number of '\texttt{g}'s. The full string can be written as:

\reg{(oo)*(og)?(gg)*}

\end{multicols}
\subsection{}

\begin{multicols}{2}
\subsubsection{}
'\textit{a}'s and '\textit{b}'s are placed at the same time, using the \textit{T} starting symbol. After all '\textit{a}'s and '\textit{b}'s are placed, the symbol turns to '\textit{S}', which places at least 1 '\textit{c}'.

\[
  T =
  \begin{cases}
    aTb \\
    S
  \end{cases}
\]
\[
  S =
  \begin{cases}
    c \\
    cS
  \end{cases}
\]

\subsubsection{}
Like with the previous problem, both the '\textit{a}'s and the '\textit{b}'s are placed at the same time, but the '\textit{b}'s are placed 2 at a time.

\[
  T =
  \begin{cases}
    aTbb \\
    abb
  \end{cases}
\]

\subsubsection{}
Once again, '\textit{a}'s and '\textit{b}'s are placed at the same time with the starting symbol \textit{T}. Afterwards, any amount of '\textit{a}'s are placed before the '\textit{b}'s.

\[
  T =
  \begin{cases}
    aTb \\
    S
  \end{cases}
\]
\[
  S =
  \begin{cases}
     \\
    aS
  \end{cases}
\]
\end{multicols}

\newpage
\subsection{}
\subsubsection{}
\texttt{\%nonassoc letprec} designates the \texttt{let} token as being non-associative, which means ambiguous implementations will lead to a syntax error.

\subsubsection{}
The order of the associativity declarations provide the precedence for the operators. So in the current way the code in written, the line \texttt{let x = 10 in x + 10 > 15} will be parsed as \texttt{let x = 10 in (x < 15)}, but if , it would be parsed as \texttt{(let x = 10 in x) < 15}.

\subsubsection{}
The code \texttt{\{ Let (Dec (fst \$2, \$4, \$3), \$6, \$1) \}} creates a \texttt{Let} instance, which containes the declared variable, the following expression, as well as the keyword.

\section{}
See code.

\section{}
\subsubsection{}
\reg{filter (('a -> bool) * ['a]) -> ['a]}

\subsubsection{}

\begin{lstlisting}
CheckExp(exp, vtable, ftable) = case exp of
  filter(p, arr_exp) =>
    let array_type = CheckExp(arr_exp, vtable, ftable)
    let element_type = match array_type with
      | Array(type) -> type
      | _ -> Error()

    let function_type = lookup(ftable, name(p))
    match function_type with
      | unbound -> Error()
      | (input_type, output_type) ->
        if input_type == element_type && output_type == bool then
          Array(element_type)
        else Error()
      | _ -> Error()
\end{lstlisting}

\newpage
\section{}


\end{document}