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Annotation of researchv10no/cmd/sml/doc/refman/reference.tex, revision 1.1

1.1     ! root        1: \chapter{Reference values}
        !             2: \label{reference}
        !             3: References are cells whose contents may be changed after creation by
        !             4: assignment.  The \verb"ref" ``datatype'' constructor, and its
        !             5: corresponding value constructor, are almost as if defined by the declaration
        !             6: \begin{verbatim}
        !             7: datatype 'a ref = ref of 'a
        !             8: \end{verbatim}
        !             9: Thus, a reference whose initial contents are the string \verb|"abc"|
        !            10: may be created by \verb|val r = ref "abc"|.  Subsequently, the
        !            11: contents of \verb"r" may be altered by assignment: \verb|r := "def"|.
        !            12: The contents of a reference may be examined by using the \verb"ref"
        !            13: constructor in a pattern:
        !            14: \begin{verbatim}
        !            15: let val (ref s) = r
        !            16:  in print s
        !            17: end
        !            18: \end{verbatim}
        !            19: The function \verb"!" is defined to take the contents of a reference;
        !            20: that is,
        !            21: \begin{verbatim}
        !            22: fun ! (ref x) = x
        !            23: \end{verbatim}
        !            24: 
        !            25: References are not fully polymorphic; see Chapter~\ref{reftype}.
        !            26: 
        !            27: Formally, we say that phrases in ML are evaluated in the presence of
        !            28: an {\em environment} $E$ and a {\em store} $S$.  The effect on $E$ of
        !            29: evaluating declarations, expressions, etc.  is described in
        !            30: Chapter~\ref{eval}.  Here we summarize the effect on $S$.
        !            31: 
        !            32: The store $S$ maps reference values to their contents.  Evaluation of
        !            33: an expression in the store $S$ yields, depending on the form of the
        !            34: expression,
        !            35: \begin{description}
        !            36: \item[\verb"ref" exp\hfill] exp is evaluated in $S$,
        !            37: producing a value $v$ and a store $S'$; the 
        !            38: reference value $r$ is returned with the store $S'+\{r \mapsto v\}$.
        !            39: 
        !            40: \item[${\rm exp}_1$~${\rm exp}_2$\hfill]  ${\rm exp}_1$ is evaluated in $S$
        !            41: yielding the function $f_1$ and store $S'$;
        !            42: ${\rm exp}_2$ is evaluated in $S'$ yielding $v_2$ and $S''$;
        !            43: finally the body of $f_1$ is evaluated with its variable bound to $v_2$,
        !            44: in the store $S''$, yielding the result $v$ and the store $S'''$.
        !            45: 
        !            46: \item[\verb"op := "$({\rm exp}_1,{\rm exp}_2)$\hfill]  The expression 
        !            47: $({\rm exp}_1,{\rm exp}_2)$ is evaluated in $S$, yielding the pair
        !            48: $(r,v)$ and the store $S'$; then the unit value \{\} is returned with
        !            49: the store $S'+\{r \mapsto v\}$.
        !            50: 
        !            51: \item[\protect\verb"\{" ${\rm lab}_1$ \protect\verb"=" ${\rm exp}_1$ , \underline{\ \ \ } , ${\rm lab}_n$ \protect\verb"=" ${\rm exp}_n$ \protect\verb"\}" \hfill]  
        !            52: ${\rm exp}_1$ is evaluated in $S$, yielding $v_1$ and the store
        !            53: $S_1$; then each ${\rm exp}_i$ is evaluated in $S_{i-1}$, yielding $v_i$ and the store $S_i$; then the record
        !            54: $\{ {\rm lab}_1 = v_1 , ... , {\rm lab}_n = v_n \}$ is returned with
        !            55: the store $S_n$.  Note that the expressions are evaluated in the
        !            56: sequence they are written, not in alphabetical order of the labels.
        !            57: 
        !            58: \item[\protect\verb"raise" exp\hfill]  exp is evaluated in $S$, returning $v$
        !            59: and $S'$; then the exception-packet $(v,S')$ is raised.
        !            60: 
        !            61: \item[exp \verb"handle" match\hfill]  exp is evaluated; if exp returns a
        !            62: value $v$ with state $S'$, then $v$ is returned with $S'$.  
        !            63: If exp raises an exception-packet $(e,S'')$ then
        !            64: the match is applied to $e$ in the state $S''$.
        !            65: If the match fails, then $(e,S'')$ is raised
        !            66: (as the value of the \verb"handle" expression).  If the match
        !            67: succeeds, then the resulting value is returned.
        !            68: \end{description}
        !            69: 
        !            70: Matching a pattern to a value has no effect on the store.  Evaluating
        !            71: a value binding has an effect on the store just from the evaluation of
        !            72: the constituent expressions.  Evaluation of type, datatype, or exception
        !            73: bindings has no effect on the store.
        !            74: 

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