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\section{Label Expressions}
A \newdef{label expression} is a constant
expression defined in terms of labels, or user
defined \href{constants.html}{constants}. MLRISC uses the type
\sml{labexp} to represent label expressions. Label expressions
are defined in the structure
\mlrischref{instructions/labelExp.sml}{LabelExp}.
The datatype \sml{labexp} has the following definition:
\begin{SML}
datatype labexp =
LABEL of Label.label
| CONST of Constant.const
| INT of int
| PLUS of labexp * labexp
| MINUS of labexp * labexp
| MULT of labexp * labexp
| DIV of labexp * labexp
| LSHIFT of labexp * word
| RSHIFT of labexp * word
| AND of labexp * word
| OR of labexp * word
\end{SML}
In addition, the following functions are defined in \sml{labexp}:
\begin{itemize}
\item \sml{valueOf : labexp -> int} -- Returns the value associated with
a label expression
\item \sml{toString : labexp -> string} -- Return the pretty printed representation of an expression
\item \sml{hash : labexp -> word} -- Returns the hash value of an expression
\item \sml{== : labexp * labexp -> bool} -- Tests whether two label expression are lexically identical
\end{itemize}
The type \sml{labexp} is depends on client defined
\href{constants.html}{constants} typed. The functor \sml{LabelExp}
is parameterized as follows.
\begin{SML}
functor \mlrischref{instructions/labelExp.sml}{LabelExp}(Constant : \mlrischref{instructions/constant.sig}{CONSTANT})
\end{SML}
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