11. Piccola
A Small Composition Language
PS — Piccola
Roadmap
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>
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>
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Applications = Components + Scripts
Piccola layers
Forms + Agents + Channels
Wrapping components
Status: semantics, types …
© 2003, Oscar Nierstrasz
1.2
PS — Piccola
Roadmap
>
>
>
>
>
Applications = Components + Scripts
Piccola layers
Forms + Agents + Channels
Wrapping components
Status: semantics, types …
© 2003, Oscar Nierstrasz
1.3
PS — Piccola
Applications = Components + Scripts
Components
both import
and export
services
Scripts plug
components
together
© 2003, Oscar Nierstrasz
1.4
PS — Piccola
Towards a Composition Language
Scripting
languages
Configure applications
from components
Coordination
languages
ADLs
Specify architectural
styles
Glue languages
Adapt components
Configure applications
from distributed services
© 2003, Oscar Nierstrasz
1.5
PS — Piccola
Roadmap
>
>
>
>
>
Applications = Components + Scripts
Piccola layers
Forms + Agents + Channels
Wrapping components
Status: semantics, types …
© 2003, Oscar Nierstrasz
1.6
PS — Piccola
Piccola — A Small Composition
Language
Piccola agents compose and coordinate external
components
Define styles
Script connections
Scripts
Agents
Coordinate activities
Forms
Channels
Adapt protocols
Components
© 2003, Oscar Nierstrasz
1.7
PS — Piccola
Piccola Layers
Applications
Compositional styles
Standard libraries
components + scripts
streams, events, GUIs, ...
basic coordination
abstractions, built-in types
Piccola
operator syntax,
introspection, component
wrappers
Piccola-calculus
forms, agents, channels,
services
© 2003, Oscar Nierstrasz
1.8
PS — Piccola
Roadmap
>
>
>
>
>
Applications = Components + Scripts
Piccola layers
Forms + Agents + Channels
Wrapping components
Status: semantics, types …
© 2003, Oscar Nierstrasz
1.9
PS — Piccola
Forms + Agents + Channels
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Forms embody structure
— immutable, extensible records
— they unify components, services and namespaces
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Agents embody behaviour
— concurrently executing scripts
— they unify concurrency and composition
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Channels embody state
— mailboxes for communicating agents
— unify synchronization and communication.
© 2003, Oscar Nierstrasz
1.10
PS — Piccola
Piccola in a Nutshell
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A form consists of a sequence of bindings:
helloForm =
text = "hello world "
do : println text
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# a form
# a binding
# a service
A script composes components by invoking services
makeFrame
title = "AWT Demo "
component =
Button.new(helloForm) ? ActionPerformed
helloForm.do
>
High-level connectors hide details of object wiring.
© 2003, Oscar Nierstrasz
1.11
PS — Piccola
Hello World
© 2003, Oscar Nierstrasz
1.12
PS — Piccola
Piccola Syntax
Values are bound with “=”, services with “:”
a
b
c
d
=
=
=
=
1
(x=1, y=2)
(b, z=3)
c.z
f: a
println f()
g u: u.x + u.y
println g(b)
h u v: u + v
println (h 1 2)
© 2003, Oscar Nierstrasz
#
#
#
#
1
3
3
binding
nested binding
extension
projection
# service (no arg)
# application (empty form)
# service
# application
# curried service
# curried application
1.13
PS — Piccola
Indentation
Complex forms can be expressed by multiple indented lines
b =
x = 1
y = 2
# nested form
println
g
b
y=b.x
#
#
#
#
2
invoke println with arg…
invoke g with arg …
argument to g
extended with new binding
Cf. Haskell and Python
© 2003, Oscar Nierstrasz
1.14
PS — Piccola
Fixpoints
Recursive services and forms are specified with the def keyword
def fib n:
if n<=2
then: 1
else: fib(n-1) + fib(n-2)
# naïve fibonacci
Note that if is a curried service …
© 2003, Oscar Nierstrasz
1.15
PS — Piccola
Control structures
>
Control structures are defined as standard Piccola
services
if bool cases:
'cases=(then:(),else:(),cases)
bool.select(true=cases.then, false=cases.else)()
© 2003, Oscar Nierstrasz
1.16
PS — Piccola
Local bindings and defaults
Bindings are made private with the ' operator
Defaults are defined by extension
if bool cases:
'default =
then:()
else:()
'cases = (default, cases)
'doit = bool.select
true = cases.then
false = cases.else
doit()
© 2003, Oscar Nierstrasz
# define defaults
# set defaults
# select service
# invoke it
1.17
PS — Piccola
Agents and Channels
You can start a concurrent agent by invoking run, passing it a form with
a do service.
run
do: …
# do something time-consuming
A channel is a mailbox for communicating with agents:
c = newChannel()
run (do: println c.receive())
c.send("hello!")
# create a channel
# start an agent
# send it a message
hello!
© 2003, Oscar Nierstrasz
1.18
PS — Piccola
Infix and prefix operators
Services may be defined as infix or prefix operators:
newVar X:
'c = newChannel()
'c.send(X)
*_:
'val=c.receive()
'c.send(val)
val
_<-_ X:
'val=c.receive()
'c.send(X)
X
x = newVar(1)
println (*x)
1
println (x<-2)
2
NB: Without agents and channels, Piccola is purely functional
© 2003, Oscar Nierstrasz
1.19
PS — Piccola
Example: Futures
Agents and channels can be used to define common coordination
abstractions, like futures, semaphores, readers/writers
synchronization policies and so on …
A “future” is a ticket for a value that is still being computed:
future task:
'result = newChannel()
'run(do: result.send(task.do()))
result.receive
f12 = future(do: fib(12))
println f12()
144
© 2003, Oscar Nierstrasz
1.20
PS — Piccola
Explicit namespaces
Everything is a form,
including the current
namespace, known as
root.
'x=1
Is just sugar for:
root=(root,x=1)
© 2003, Oscar Nierstrasz
1.21
PS — Piccola
Reflection
Piccola provides various built-in services to inspect and manipulate forms
countLabels f:
'count = newVar(0)
'forEachLabel
form = f
do L: count <- 1 + *count
*count
# a standard service
println (countLabels root)
75
© 2003, Oscar Nierstrasz
1.22
PS — Piccola
Roadmap
>
>
>
>
>
Applications = Components + Scripts
Piccola layers
Forms + Agents + Channels
Wrapping components
Status: semantics, types …
© 2003, Oscar Nierstrasz
1.23
PS — Piccola
Language bridging
Piccola does not provide booleans, numbers or strings — instead,
these are forms that wrap Host language objects as components
— Host objects are automatically wrapped as forms when imported to
Piccola, and unwrapped when Host services are invoked
— Special wrappers may be registered to adapt the interfaces of selected
components
© 2003, Oscar Nierstrasz
1.24
PS — Piccola
Wrapping AWT components
Java AWT Frames are automatically wrapped to add a ? connector:
def wrapComponent X:
...
_?_ aHandler aClosure:
...
...
wrapper.frame X:
wrapComponent X
...
registerWrapper "java.awt.Frame" wrapper.frame
© 2003, Oscar Nierstrasz
1.25
PS — Piccola
Roadmap
>
>
>
>
>
Applications = Components + Scripts
Piccola layers
Forms + Agents + Channels
Wrapping components
Status: semantics, types …
© 2003, Oscar Nierstrasz
1.26
PS — Piccola
Other features
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Module system based on explicit namespaces
Anonymous services (lambdas)
Dynamic scoping on demand
A simple testing framework
Lists, Sets and Maps
© 2003, Oscar Nierstrasz
1.27
PS — Piccola
Piccola Semantics
The semantics of Piccola is
defined using a process
calculus with first-class
namespaces
© 2003, Oscar Nierstrasz
1.28
PS — Piccola
How forms support composition
© 2003, Oscar Nierstrasz
1.29
PS — Piccola
Results so far
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Piccola implementations for Java and Squeak
— Sophisticated language bridging
— Efficient partial evaluation
— Various experimental composition styles
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Formal semantics
— π calculus with first-class namespaces
— Limited reasoning about styles and composition
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Publications and software: www.iam.unibe.ch/~scg
© 2003, Oscar Nierstrasz
1.30
PS — Piccola
What’s missing?
Explicit components, connectors & styles
> Type-checking provided and required services
> Reasoning about styles and compositions
> Adapting/reengineering existing services
>
© 2003, Oscar Nierstrasz
1.31
PS — Piccola
The future of composition?
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Dynamic composition of ubiquitous services
Service discovery and negotiation
Reconfiguration validation and run-time monitoring
New, fine-grained models of components, composition and
evolution
Component models for mobile, pervasive and embedded systems
© 2003, Oscar Nierstrasz
1.32
PS — Piccola
License
>
http://creativecommons.org/licenses/by-sa/2.5/
Attribution-ShareAlike 2.5
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• to copy, distribute, display, and perform the work
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Under the following conditions:
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© 2003, Oscar Nierstrasz
1.33