Continue Spec

doc/spec/03.03.scope_expressions.tex

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+\begin{figure}[t]
+  \[
+    \begin{array}{rcl}
+      \defterm{scopeExpression}                & : & \nonterm{definition}^\star\s\nonterm{expression}\\
+      \defterm{definition}                     & : & \nonterm{variableDefinition}\alt\nonterm{functionDefinition}\alt\nonterm{infixDefinition}\\
+      \defterm{variableDefinition}             & : & (\s\term{local}\alt\term{public}\s)\s\nonterm{variableDefinitionSequence}\s\term{;}\\
+      \defterm{variableDefinitionSequence}     & : & \nonterm{variableDefinitionSequenceItem}\s(\s\term{,}\s\nonterm{variableDefinitionSequenceItem}\s)^\star\\
+      \defterm{variableDefinitionSequenceItem} & : & \token{LIDENT}\s[\s\term{=}\s\nonterm{basicExpression}\s]\\
+      \defterm{functionDefinition}             & : & [\s\term{public}\s]\s\term{fun}\s\token{LIDENT}\s\term{(}\s\nonterm{functionArguments}\s\term{)}\s\nonterm{functionBody}\\
+      \defterm{functionArguments}              & : & [\s\token{LIDENT}\s(\s\term{,}\s\token{LIDENT}\s)^\star\s]\\
+      \defterm{functionBody}                   & : & \term{\{}\s\nonterm{scopeExpression}\s\term{\}}
+    \end{array}
+  \]
+  \caption{Scope expression concrete syntax}
+  \label{scope_expression}
+\end{figure}
+
+\section{Scope Expressions}
+
+Scope expressions provide a mean to put expressions is a scoped context. The definitions in scoped expressions comprise of function definitions and
+variable definitions (see Fig.~\ref{scope_expression}). For example:
+
+\begin{lstlisting}
+    local x, y, z; -- variable definitions
+
+    fun id (x) {x} -- function definition
+\end{lstlisting}
+
+As scope expressions are expressions, they can be nested:
+
+\begin{lstlisting}
+    local x;
+
+    { -- nested scope begins here
+      local y;
+      skip
+    } -- nested scope ends here
+\end{lstlisting}
+
+The definitions on the top-level of compilation unit can be tagged as ``\lstinline|public|'', in which case they are exported and become visible by
+other units which import the given one. Nested scopes can not contain public definitions.
+
+The nesting relation has the shape of a tree, and in a concrete node of the tree all definitions in all enclosing scopes are visible:
+
+\begin{lstlisting}
+    local x;
+
+    {local y; 
+      {local z;
+        skip -- x, y, and z are visible here
+      };
+      {local t;
+        skip -- x, y, and t are visible here
+      };
+      skip -- x and y are visible here
+    };
+    skip -- only x is visible here
+\end{lstlisting}
+
+Multiple definitions of the same name in the same scope are prohibited:
+
+\begin{lstlisting}
+    local x;
+    fun x () {0} -- error
+\end{lstlisting}
+
+However, a definition is a nested scope can override a definition in an enclosing one:
+
+\begin{lstlisting}
+    local x;
+
+    {
+      fun x () {0} -- ok
+      skip         -- here x is associated with the function
+    };
+
+    skip -- here x is asociated with the variable
+\end{lstlisting}
+
+A function can freely use all visible definitions; in particular, functions defined in the
+same scope can be mutually recursive:
+
+\begin{lstlisting}
+    local x;
+    fun f () {0}
+
+    { 
+      fun g () {f () + h () + y} -- ok
+      fun h () {g () + x}        -- ok
+      local y;
+      skip
+    };
+    skip
+\end{lstlisting}
+
+A variable, defined in a scope, can be attributed with an expression, calcualting its initial value.
+These expressions, however, are evaluated in the order of variable declaration. Thus, while
+technically it is possible to have forward references in the initialization expression, their
+behaviour is undefined. For example:
+
+\begin{lstlisting}
+    local x = y + 2; -- undefined, as y is not yet initialized at this point
+    local y = x + 2;
+    skip
+\end{lstlisting}