Flow Control

Closure

A closure is a function that references variables bound in its lexicalenvironment.

Depending on the language, it may or may not be able to changetheir values.

A variable may therefore exist even outside of its scope.

Michal Pıse (CTU in Prague) Object Programming Lect. 8: C3 November 16, 2010 2 / 10

Anonymous Classes in Java

public static void main(String[] args) {final JFrame frame = new JFrame();

frame.addWindowListener(new WindowAdapter() {public void windowClosing(WindowEvent e) {frame.dispose();

}});

frame.setVisible(true);

}


Implementation of Closures

One object per closure.

One object representing shared state.

Invisible accessors.


Coroutine

A coroutine is a subroutine generalization with multiple entry pointsfor suspending and resuming execution at certain locations.

Invocation of a coroutine is not stateless—context andenvironment are withheld.


Example

class Set {int[] contents;

...

class SetIterator {int next() {for (int i = 0; i < contents.length; i++)

yield contents[i];

throw new RuntimeException();

}...

}}


Continuation

When a program is written using continuation-passing style, itsfunctions never return.

Instead, they invoke functions that represent the rest of thecomputation (continuations).

It can be viewed as a goto statement with parameters.


Continuation

When a program is written using continuation-passing style, itsfunctions never return.

Instead, they invoke functions that represent the rest of thecomputation (continuations).

It can be viewed as a goto statement with parameters.


Example

data Regexp = Character Char | Concat Regexp Regexp

m :: Regexp -> Maybe String ->

(Maybe String -> Maybe String) -> Maybe String

m regex Nothing cont = cont Nothing

m (Character c) (Just "") cont = cont Nothing

m (Character c) (Just (first:rest)) cont

| c == first = cont (Just rest)

| otherwise = cont Nothing

m (Concat r1 r2) str cont = m r1 str

(\param -> m r2 param cont)

match :: Regexp -> String -> Bool

match regexp str = (m regexp (Just str) id) == Just ""


Example (II)

data Regexp = Character Char | Concat Regexp Regexp |

Altern Regexp Regexp

m (Altern r1 r2) str cont

m r1 str (\param -> if cont param /= Just ""

then m r2 str cont

else cont param)


Example (II)

data Regexp = Character Char | Concat Regexp Regexp |

Altern Regexp Regexp

m (Altern r1 r2) str cont

m r1 str (\param -> if cont param /= Just ""

then m r2 str cont

else cont param)


See

Hayo Thielecke. Continuations, functions and jumps. SIGACT News30, 2 (June 1999). 33–42.http://doi.acm.org/10.1145/568547.568561


Education

Flow Control