\part{Type system}

An Elna program consists of one or more source files, called \textbf{modules}.

Each module can declare \textbf{types}, \textbf{global variables} and
\textbf{procedures}, used by this module or exported to be used by other
modules.

\begin{grammar}
<type> = <array-type>
	\alt{} <pointer-type>
	\alt{} <record-type>
	\alt{} <enumeration-type>
	\alt{} <procedure-type>
	\alt{} <identifier>.
\end{grammar}

\chapter{Primitive types}

\begin{itemize}
	\item Pointer
	\item Word
	\item Int
	\item Bool
	\item String
	\item Char
\end{itemize}

\chapter{Pointer types}

\begin{grammar}
<pointer-type> = `^' <type>.
\end{grammar}

\begin{lstlisting}[caption=Example]
var
  x: Int;
  y: ^Int;
begin
  y := @x;
  y^ := 0
end.
\end{lstlisting}

\chapter{Static array}

\begin{grammar}
<array-type> = `[' <expression> `]' <type>.
\end{grammar}

\begin{lstlisting}[caption=Example]
var
  array: [3]Int := [1, 2, 3];
begin
  array[1] := array[2]
end.
\end{lstlisting}

\chapter{Procedure types}

\begin{grammar}
<identifiers> = <identifier> \{`,' <identifier>\}.

<parameter> = <identifiers> `:\@' <type>.

<procedure-heading> = `(' [<parameter> \{`;' <parameter>\}] `)' <return-declaration>.

<procedure-body> = <constant-part> <variable-part> <statement-part> [`return' <expression>] `end'.

<procedure-declaration> = `proc' <identifier-definition> <procedure-heading> \\
	(<procedure-body> | `extern').

<return-declaration> = [`:\@' `!\@' | `:\@' type].

<procedure-type> = `proc' <procedure-heading>.
\end{grammar}

\begin{lstlisting}[caption=Example]
var
  a: proc(Int) -> Int;

proc f(x: Int) -> Int;
end;

begin
  a := f;
  a(0)
end.
\end{lstlisting}

\chapter{Records}

\begin{grammar}
<field> = <identifiers> `:\@' <type>.

<record-type> = `record' [`(' <identifier> `)'] [<field> \{`;' <field>\}] `end'.
\end{grammar}

\begin{lstlisting}[caption=Example]
type
  T = record
    x: Int
  end;
  U = record(T)
    y: Int;
    z: Int
  end;

var
  u: U;
begin
  u := U{x: 0, y: 1, z: 2};
  u.x := 3
end.
\end{lstlisting}

\chapter{Enumerations}

\begin{grammar}
<enumeration-type> = `(' <identifiers> `)'.
\end{grammar}

\begin{lstlisting}[caption=Example]
type
  E = (one, two, three);
var
  e: E;
begin
  e := E.one
end.
\end{lstlisting}