slide1 n.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
μ Icon PowerPoint Presentation
Download Presentation
μ Icon

Loading in 2 Seconds...

play fullscreen
1 / 13

μ Icon - PowerPoint PPT Presentation


  • 143 Views
  • Uploaded on

μ Icon. ebbesen@itu.dk. Bullets on parade. Preliminary: Continuation passing style. The code following a function call is wrapped up in a method - the continuation - and passed along as a parameter. Calculating the n-th factorial recursively In continuation passing style

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'μ Icon' - gitel


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
slide1

μIcon

ebbesen@itu.dk

Bullets on parade

preliminary continuation passing style
Preliminary: Continuation passing style
  • The code following a function call is wrapped up in a method - the continuation - and passed along as a parameter.
  • Calculating the n-th factorial recursively
  • In continuation passing style
  • Actually it’s the closure we’re passing along, so this style of programming restricts our choise of language (ML/C#/Java?), leaving out e.g. C and Object Pascal.

fun fac 0 = 1

| fac n = n * fac (n - 1)

fun fac (k, 0) = k 1

| fac (k, n) = fac (fn x => k (n * x), n - 1)

n-th factorial = fac(fn x => x, n)

icon a crash course i
Icon - A crash course I
  • Whereas C, C++, Java and other related languages differentiate between statements and expressions, Icon has only expressions.
  • Icon expressions may either fail or succeed, and successful expression may generate not only one, but a (not necessarily finite) sequence of results.
  • A successful expression may produce either values or variables.

(1 < 0) # failing

"hi world" # generating a single value

(0 to 9) # generating 0, 1, 2, ..., 9

(7 | 9 | 13) # generating 7, 9, 13

(x := 7) # generating the variable x

(x | y | z) # generating x, y, z

e.g. every ((x | y | z) := 1)

icon a crash course ii
Icon - A crash course II
  • It's perfectly allowable for an expression to fail.
  • Icon provides many of the well-known control structures (if-then-else, while-do, etc.), where instead of boolean values, the success or failure of an evaluation determines control flow.
  • Failure is propagated through operators/functions, and thus the failure mechanism acts much like C#/Java exceptions.

while line := read() do # read() fails on

process(line) # reaching EoF

while process(read()) # like above

# neat isn’t it

icon a crash course iii
Icon - A crash course III
  • Expression evaluation is goal-directed, in the sense that an outer expression will resume its inner subexpressions in an attempt to succeed.
  • Inner expressions are resumed from right to left why we use the term backtracking.

write((i := (0 to 9)) & (3 < i))

# eventually succeeds, writing out 4

(7 < write(y <- (0 to 9)))

# writes out "0 1 2 3 4 5 6 7 8" before

# eventually succeeding, generating 8

icon a crash course iv a few oddities
Icon - A crash course IV (a few oddities)
  • Unlike their cousins in C/C++/Java, Icon variables are typeless. Values however are typed.
  • Icon variables may carry any type of value, and type conversion of the variable value will be performed automatically as required.
  • Icon variable are late bound, i.e. dereferenced only when the value is needed.
  • Procedure arguments are passed by value, however due to the late binding of variables, dereferencing is not done until all arguments have been evaluated (arguments are evaluated from left to right, C/C++ make no such promises):

fun(x := 1, x := x + 1);

# calls fun with the arguments (2, 2)

slide7
μIcon
  • All μIcon values are of integer type.
  • Regular Icon procedures are first class citizens, and may be assigned to variables, passed as arguments, etc. μIcon procedures are not, but are similar to C functions.
  • Only local variables are supported, global variables are not.
  • μIcon includes only a small subset of the full set of Icon expressions.
  • μIcon procedures have so far accepted only a single argument. Today we'll add support for multiple-argument procedures.
the inner workings of the interpreter i data
The inner workings of the interpreter I (data)
  • [Griswold] “The term outcome is used to describe the result of evaluating an expression, whether it is success or failure.”
  • [Griswold] “The term result is used collectively to include both values and variables.”

+----------+

| Outcome |

+----------+

^ ^

| |

| +----------+

| | Failure |

| +----------+

|

+----------+ +----------+

| Success |<>---->| Result |

+----------+ +----------+

^ ^

| |

| +----------+ +----------+

| | Variable |<>---->| Store |<>---+

| +----------+ +----------+ |

| |

+----------+ |

| Value |<----------------------------------+

|----------|

| int: i |

+----------+

the inner workings of the interpreter ii
The inner workings of the interpreter II
  • All expressions implement the Expression interface, defining a single method Eval taking as input four continuations and a run-time environment.
  • If the expression succeeds, we'll call the success continuation cont passing along the produced result as well as a backtracking continuation, allowing an enclosing expression to ask for more results.
  • The second argument fcont is the failure/backtracking continuation, to be called if the expression fails.

public interface Expression

{

OutcomeEval(Cont cont, Fcont fcont, REnv env,

Cont rc, Fcont rfc);

}

multiple argument procedures i
Multiple-argument procedures I
  • Let’s have a look at the source code (you might want to bring along a cup of coffee).
a few perverted examples
A few perverted examples
  • The late binding of variables:
  • Generating a new set of arguments if the body fails:

procedure sum(a, b) # writes 2 to

return (a + b) # standard out

end

procedure main ()

x := 0;

write(sum(x, x := x + 1))

end

procedure backtrack(i, j) # writes "0 1 2 3 4"

return 3 < write(j) # to standard out

end

procedure main ()

backtrack(x := (0 to 9), y := x)

end

concluding thoughts highly subjective
Concluding thoughts (highly subjective)
  • Think twice before introducing continuations and apply with care, as the readability of your code will suffer greatly (did anyone spot the error in the report?)
  • While the goal directed evaluation of expressions is an intriguing idea, I’ve yet to come across a problem where this brute force approach would the best answer.
  • Honestly, I can't for the love of God see how the late dereferencing of arguments will lead to anything but trouble.
  • I do appreciate how making failure of expressions an integrated part of the language allows for very elegant code (a kind of lightweight exceptions).
  • Keep in mind though that you’ll only be able to appreciate the value of an answer once you know the right question…