/ e-impls / e-on-c

Compiling Expressions

E is an expression language, but, unfortunately, C++ is not. To deal with this mismatch, we compile E using one of two functions:

• FOR_VALUE(EExpr) => resultVarName; outputs C++ statements
• FOR_EFFECT(EExpr); outputs C++ statements

DefineExpr

e`def @var :@slotMakerExpr := @expr`

C++

FOR_EFFECT

def resultName := FOR_VALUE(expr)
MUST_MATCH(patt, resultName)

FOR_VALUE

def a := c + d

Ref result := c.call(&DoAdd, d);
Ref a = result;

e`def @patt := @expr`

C++

FOR_EFFECT

def resultName := FOR_VALUE(expr)
DECLARE_TEMPS(patt)
MUST_MATCH(patt, resultName)
ACTUALS_FROM_TEMPS(patt)

FOR_VALUE

def [a, b] := c + d

Ref result := c.call(&DoAdd, d);
Ref aTemp;
Ref bTemp;
ListMustMatch(&aTemp, &bTemp, result);
Ref a = aTemp;
Ref b = bTemp;

"def" patt ":=" rValue.

Match patt against the value of rValue.  Neither patt nor rValue may
use any variables defined by the other.  This allows their scopes to
be order independent.
 

//If patt is a FinalPattern, then it works according to the "def"
//rules above.  Otherwise match against temporary variables.  If match
//succeeds, initialize the actual local variables to the temporaries.
//Anywhere it fails, throw the (XXX to-be-specified) suitable
//exception.  Since the actual variable would then never come into
//scope, they need not be defined and initialized.
 
 

CallExpr

e`@recip.@verb(@arg0,...)`

C++

FOR_EFFECT

def recipName := FOR_VALUE(recip)
def arg0Name := FOR_VALUE(arg0)
...
cpp`$recipName.call(&$DoVerb, $arg0Name, ...);`

FOR_VALUE

def recipName := FOR_VALUE(recip)
def arg0Name := FOR_VALUE(arg0)
...
def resultName := newTemp()
cpp`Ref $resultName := $recipName.call(&$DoVerb, $arg0Name, ...);`

a + b

Ref result = a.call(&DoAdd, b);

Eval left to right, then synchronously call the verb method of the value of recip with the values of the args.

SendExpr

e`@recip <- @verb(@arg0,...)`

C++

FOR_EFFECT

def recipName := FOR_VALUE(recip)
def arg0Name := FOR_VALUE(arg0)
...
cpp`$recipName.sendOnly(&$DoVerb, $arg0Name, ...);`

FOR_VALUE

def recipName := FOR_VALUE(recip)
def arg0Name := FOR_VALUE(arg0)
...
def resultName := newTemp()
cpp`Ref $resultName := $recipName.send(&$DoVerb, $arg0Name, ...);`

a <- add(b)

Ref result = a.send(&DoAdd, b);

Like call, but asks that recip do the operation <i>eventually</i>, in its own turn.  

EscapeExpr

e`escape @hatch { @body }`

C++

FOR_EFFECT

FOR_VALUE

def result := escape break {
    loop {
        if (foo()) {
            break(x)
        }
    }
}

Ref result;
Ejector * ejector = new Ejector();
Ref break = ejector.ref();
try {
    while(true) {
        if (test(foo.call(&DoRun))) {
            break.call(&DoRun, x);
        }
    }
} catch (Ejection *ej) {
    result = ejector->result(ej);
} finally {
    ejector->disable();
}

Bind hatch to an escape hatch.  If the escape hatch's run/1 is called during the execution of body, the escape expression will be exited early, and the run argument will be the value.  If run/0 is called, it's as if run(null) were called.

HideExpr

e`{ @ body }`

Evaluate body, but hide all variable names it defines from the surrounding scope.  

Only has an effect on the compiler's scope calculation.  Generates no code.

IfExpr

e`if (@query) { @then } else {@els }`

C++

FOR_EFFECT

def queryName := FOR_VALUE(query)
cpp`if (test($queryName)) {
    ${FOR_EFFECT(then)}
} else {
    ${FOR_EFFECT(els)}
}

FOR_VALUE

def queryName := FOR_VALUE(query)

def resultName := newTemp()
cpp`Ref $resultName;
if (test($queryName)) {
    ${def name0 := FOR_VALUE(then)}
    cpp`$resultName = $name0;
} else {
    ${def name1 := FOR_VALUE(els)}

    cpp`$resultName = $name1;
}

Evaluate test to a boolean.  If true, the value is the evaluation of then.  Else the value is the evaluation of els.

Not yet dealt with: then evaluates in the scope of query, els does not.

LiteralExpr

This value is the value of the expression.
 

'c'

Ref('c')

"foo"

Ref("foo")

true

Ref(true)

false

Ref(false)

1234

Ref(1234)

1000000000000

BigInteger.call(&DoFromString,
                Ref("1000000000000"))

3.14159

Ref(3.14159)

LoopExpr

e`loop { @body }`

C++

FOR_EFFECT

cpp`while (true) {
    ${FOR_EFFECT(body)}
}

FOR_VALUE

Execute body repeatedly forever (or until stopped by other means).

Not yet formatted, edited, or made readable

But does contain useful content. Proceed past this point at your own risk.

MatchBindExpr

specimen "=~" patt

Match patt against the value of specimen.  Say whether it matches.
 

Ref specimen = TRANSLATE(specimen);
boolean result = true;
Ref local1;
...
do {
    //match specimen against patt, binding to temporary variables as
    //we go.  Anywhere we fail, we do:
        result = false;
 break;
    //After all matching steps are completed, we fall through to
    //binding the actual variables:
    local1 = temp1;
    ...
} while(false);
if (!result) {
    local1 = local2 = ... = //appropriate broken ref
}
//result is value of MatchBindExpr
 
 

ScopeExpr

"meta" "scope"

Reifies the current runtime scope as a runtime scope object.

Object visitScopeExpr();
 
 

SeqExpr

first "\n" second

Do first, then evaluate to the result of second.
 

TRANSLATE(first);
TRANSLATE(second);
 
 

MetaExpr

"meta" "(" varName ")"

varName must be the paramName of an enclosing object-expression or
plumbing-expression.  This returns the EMeta giving meta-level
access to that object.

Object visitMetaExpr(NounExpr noun);
 
 

CatchExpr

"try" "{" attempt "}" "catch" patt "{" catcher "}"

Evaluate attempt.  If it completes successfully, then its value is the
value of the catch-expression.  If it throws an exception, match it
against patt.  If it succeeds, evaluate to the evaluation of catcher.
Otherwise rethrow the exception.
 

Ref result;
try {
    result = TRANSLATE(attempt);
} catch (Ref &problem) {
    if (TRANSLATE(`$problem =~ $patt`)) {
 result = TRANSLATE(catcher);
    } else {
 throw problem;
    }
}
//result is value of Catch expression
 

FinallyExpr

"try" "{" attempt "}" "finally" "{" unwinder "}"

Evaluate attempt.  On the way out, whether by successful completion or
abrupt termination (throw or escape), in all cases evaluate the
unwinder before leaving.  If attempt succeeds, then the value of the
finally-expression is the value of attempt.
 

Ref result;
try {
    result = TRANSLATE(attempt);
} finally {
    TRANSLATE(unwinder);
}
//result is value of Finally expression