The Metacircular Evaluator $($Racket Language$)$

Implement let$*$ expressions.

Pay careful attention to the following details of the semantics of let$*$:

The expressions that determine the values of the variables bound by the let$*$ must be evaluated.

The body of a let$*$ is a sequence of expressions.

If you are unclear on the semantics of let$*$:

Read the R$5$RS standard or try out examples in DrRacket.

Hint: How can you desugar a let$*$ expression? That is$,$ how can let$*$ be implemented as a derived expression?

$($define $($true$?$ x$)$

$($not $($eq$?$ x false$)))$

$($define $($false$?$ x$)$

$($eq$?$ x false$))$

$($define $($make$-$procedure parameters body env$)$

$($list 'procedure parameters body env$))$

$($define $($compound$-$procedure? p$)$

$($tagged$-$list? p 'procedure$))$

$($define $($procedure$-$parameters p$)($cadr p$))$

$($define $($procedure$-$body p$)($caddr p$))$

$($define $($procedure$-$environment p$)($cadddr p$))$

$($define $($enclosing$-$environment env$)($cdr env$))$

$($define $($first$-$frame env$)($car env$))$

$($define the$-$empty$-$environment $\text{'}())$

$($define $($make$-$frame variables values$)$

$($mcons variables $($list$->$mlist values$)))$

$($define $($frame$-$variables frame$)($mcar frame$))$

$($define $($frame$-$values frame$)($mcdr frame$))$

$($define $($add$-$binding$-$to$-$frame! var val frame$)$

$($set$-$mcar! frame $($cons var $($mcar frame$)))$

$($set$-$mcdr$!$ frame $($mcons val $($mcdr frame$))))$

$($define $($extend$-$environment vars vals base$-$env$)$

$($if $(=($length vars$)($length vals$))$

$($cons $($make$-$frame vars vals$)$ base$-$env$)$

$($if $(<mtext\; id="EditorElement\_58952"></mtext><mi\; id="EditorElement\_58953">(</mi>$length vars$)($length vals$))$

$($error "Too many arguments supplied" vars vals$)$

$($error "Too few arguments supplied" vars vals$))))$

$($define $($lookup$-$variable$-$value var env$)$

$($define $($env$-$loop env$)$

$($define $($scan vars vals$)$

$($cond $(($null$?$ vars$)$

$($env$-$loop $($enclosing$-$environment env$)))$

$(($eq$?$ var $($car vars$))$

$($mcar vals$))$

$($else $($scan $($cdr vars$)($mcdr vals$)))))$

$($if $($eq$?$ env the$-$empty$-$environment$)$

$($error "Unbound variable" var$)$

$($let $(($frame $($first$-$frame env$)))$

$($scan $($frame$-$variables frame$)$

$($frame$-$values frame$)))))$

$($env$-$loop env$))$

$($define $($set$-$variable$-$value! var val env$)$

$($define $($env$-$loop env$)$

$($define $($scan vars vals$)$

$($cond $(($null$?$ vars$)$

$($env$-$loop $($enclosing$-$environment env$)))$

$(($eq$?$ var $($car vars$))$

$($set$-$mcar! vals val$))$

$($else $($scan $($cdr vars$)($mcdr vals$)))))$

$($if $($eq$?$ env the$-$empty$-$environment$)$

$($error "Unbound variable $--$ SET!" var$)$

$($let $(($frame $($first$-$frame env$)))$

$($scan $($frame$-$variables frame$)$

$($frame$-$values frame$)))))$

$($env$-$loop env$))$

$($define $($define$-$variable! var val env$)$

$($let $(($frame $($first$-$frame env$)))$

$($define $($scan vars vals$)$

$($cond $(($null$?$ vars$)$

$($add$-$binding$-$to$-$frame! var val frame$))$

$(($eq$?$ var $($car vars$))$

$($set$-$mcar! vals val$))$

$($else $($scan $($cdr vars$)($mcdr vals$)))))$

$($scan $($frame$-$variables frame$)$

$($frame$-$values frame$))))$