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\chapter{\tt{eval}}
\label{evalchapter}
The \rsixlibrary{eval} library allows a program to create Scheme
expressions as data at run time and evaluate them.
\begin{entry}{%
\proto{eval}{ expression environment}{procedure}}
Evaluates \var{expression} in the specified environment and returns its value.
\var{Expression} must be a syntactically valid Scheme expression represented as a
datum value, and \var{environment} must be an
\defining{environment}, which can be created using the {\cf
environment} procedure described below.
If the first argument to {\cf eval} is determined not to be a syntactically correct
expression, then {\cf eval} must raise an exception with condition
type {\cf \&syntax}. Specifically, if the first argument to {\cf
eval} is a definition or a splicing {\cf begin} form containing a
definition, it must raise an exception with condition type {\cf
\&syntax}.
\end{entry}
\begin{entry}{%
\proto{environment}{ import-spec \dots}{procedure}}
\domain{\var{Import-spec} must be a datum representing an
\hyper{import spec} (see report
section~\extref{report:librarysyntaxsection}{Library form}).}
The {\cf environment} procedure returns an environment corresponding
to \var{import-spec}.
The bindings of the environment represented by the specifier are
immutable: If {\cf eval} is applied to an expression that is
determined to contain an
assignment to one of the variables of the environment, then {\cf eval} must
raise an exception with a condition type {\cf\&assertion}.
\begin{scheme}
(library (foo)
(export)
(import (rnrs))
(write
(eval '(let ((x 3)) x)
(environment '(rnrs))))) \\\> {\it writes} 3
(library (foo)
(export)
(import (rnrs))
(write
(eval
'(eval:car (eval:cons 2 4))
(environment
'(prefix (only (rnrs) car cdr cons null?)
eval:))))) \\\> {\it writes} 2
\end{scheme}
\end{entry}
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