4 Dec 2001

Kestrel 1

From Patterns to Programming Languages

Matthias FelleisenNortheastern University

4 Dec 2001

Kestrel 2

Background and Experience

• Theory of PL Expressiveness• PLT Scheme (IDE, tools, Scheme)

– 200 Kloc Scheme, 200Kloc C++ (inh)– 400 downloads/day for last year

• 94/95: design patterns– use in implementation– implementing patterns in Scheme – “A Little Java, A Few Patterns”– papers on patterns for extensibility

• 96/97: Patterns in the IDE (ESOP)

4 Dec 2001

Kestrel 3

Patterns and Expressiveness

4 Dec 2001

Kestrel 4

Patterns (GoF)

• pattern name

• problem

• solution

• consequences

• adapter

• mismatch of interface

• create adapter class

• e.g., class adapters may introduce additional behavior

4 Dec 2001

Kestrel 5

Patterns (PL)

• name

• problem

• solution

• consequences

• store passing

• mismatch of interface: have FP, need state

• pass store as data structure; lookup & modify locations man.

• introduce additional behavior

4 Dec 2001

Kestrel 6

Patterns (PL: store passing)

val xcount = ref 0;

// xf : unit -> TypeGfun xf () = … xcount = !xcount + 1; G;

would like, but don’t have

val xcount = ref 0;

// xf : int -> (TypeG, int)fun xf (xcount) = … let val xcount = !xcount + 1; in (G, xcount);

here we go:

4 Dec 2001

Kestrel 7

Patterns (PL: store passing)

fun f fun f-sps

4 Dec 2001

Kestrel 8

Patterns (PL)

• name

• problem

• solution

• consequences

• continuation passin g

• mismatch of interface: have FP, need exceptions

• pass control as function

• introduce additional behavior

4 Dec 2001

Kestrel 9

Patterns (PL: continuation passing)

// xf : unit -> TypeGfun xf () = … if cond then exit(10); else G;

would like, but don’t have

// xf : (GType -> FA) -> FAfun xf (roc) = … if cond then 10; else roc(G);

here we go:

4 Dec 2001

Kestrel 10

Patterns (PL: continuation passing)

fun f fun f-cps

4 Dec 2001

Kestrel 11

Patterns (PL)

• name

• problem

• solution

• consequences

• nested blocks

• lack blocks in FPL

• apply anonymous function

• physically separate variables from values

4 Dec 2001

Kestrel 12

Patterns (PL: nested blocks)

(let ([x 10] [y 20]) … x … y … )

would like, but don’t have

((lambda (x y) … x … y … ) 10 20)

here we go:

4 Dec 2001

Kestrel 13

Patterns (PL)

• name

• problem

• solution

• consequences

• for-loop

• lack for-loop in FPL

• use recursive function

• physically separate loop variable from initial value

4 Dec 2001

Kestrel 14

Patterns (PL: nested blocks)

(for j = 10 to 20 (set! sum (+ sum j)))

would like, but don’t have

(for-each (lambda (j) (set! sum (+ sum j))) (build-list 10 (lambda (k) (+ k 10))))

here we go:

4 Dec 2001

Kestrel 15

Patterns from PL

• Landin: patterns are everywhere – some are local --> syntactic sugar– some are global --> essential constructs

• Strachey, Wadsworth: – denotational semantics & global patterns

• Felleisen (1988): – theory of expressiveness relative to a base

language

4 Dec 2001

Kestrel 16

Patterns and Language Constructs

4 Dec 2001

Kestrel 17

Pattern Programming

• Programming with patterns is good.

• With Patterns, programmers can discuss lots of code with a few words.

• Patterns suggest invariants, helping reasoning about code.

4 Dec 2001

Kestrel 18

Pattern Programming

• Programming patterns explicitly is bad.

• Programmers may “lie” about patterns.

• Pattern maintenance may break logical invariants

4 Dec 2001

Kestrel 19

The Adapter Pattern: One More Time

/* Adapter Pattern: object adapter

GraphicalOutput adapts VendorGraphics to OutputRoutines

*/

interface OutputRoutines { … }

class GraphicalOutput implements OutputRoutines { … }

class VendorGraphics { … }

two lines of comments explain 100 lines of code

4 Dec 2001

Kestrel 20

The Adapter Pattern: One More Time

/* Adapter Pattern: object adapter

GraphicalOutput adapts VendorGraphics to OutputRoutines

*/

class OutputRoutines { … }

class GraphicalOutput extends OutputRoutines { … }

class VendorGraphics { … }

it’s no longer an adaptation -- it’s implementation inheritance

4 Dec 2001

Kestrel 21

Patterns as Comments

• When patterns are comments and informal language, nobody knows whether our claims are true.

• Somebody else may change the code or the comments. – Then they are out of sync.

• Are such patterns useful?

4 Dec 2001

Kestrel 22

Patterns as Language Constructs

• local patterns vs global patterns

• local patterns via syntactic extensions

• global patterns via aspects (?)– if aspects work, we can experiment with a

pattern and see whether it is useful– if so, we should add them to the language.

4 Dec 2001

Kestrel 23

Scheme’s Macros: Principles

• Macros are transformations on the abstract syntax trees

• Macros respect the scoping (and other) information of the input ASTs.

• Macros respect the scoping information of their “host” module.

4 Dec 2001

Kestrel 24

Scheme’s Macros: Example

(define-syntax let (syntax-rules () [(let ([<x:var> <rhs:exp>] ...) <body:exp> ...) ((lambda (<x:var> ...) <body:exp> ...) <rhs:exp> ...)]))

;; use: (let ([x 10][y 20]) (printf "the variables are: ~s ~s ~n" x y) (+ x y))

4 Dec 2001

Kestrel 25

Local Patterns as Language Constructs

(define-syntax-pattern Adapter [rewrite (_ <aN> adapts <aT> to <dI> as <aV> (fields <fd> ...) (methods <md> ...))] [as (class <aN> implements <dI> (fields (<aT> <aV>) <fd> ...) (methods <md> ...))])

4 Dec 2001

Kestrel 26

Pattern Elaboration

aN

aT

d I

aV

m d

m eth od s

fd

fie ld s

A d ap te r

aN

d I

aT aV

fd

fie ld sm d

m eth od s

c lass

4 Dec 2001

Kestrel 27

Patterns and Synthesized Information

(Patterns and Proofs)

4 Dec 2001

Kestrel 28

Making Patterns as Constructs Work

Programmers need to see information at the level at which they program.

Information is: syntax error type error run-time exception reporting

4 Dec 2001

Kestrel 29

Information about Programs Matters

MultiplicationExpression<class LazyBinaryExpression<class AdditionExpression<class MatrixICCL::Matrix<class MatrixICCL::BoundsChecker<class MatrixICCL::ArrFormat<class MatrixICCL::StatExt<struct MatrixDSL::int_number<int,7>,struct MatrixDSL::int_number<int,7>>,class MatrixICCL::Rect<class MatrixICCL::StatExt<struct MatrixDSL::int_number<int,7>,struct MatrixDSL::int_number<int,7>>>,class MatrixICCL::Dyn2DCContainer<class MATRIX_ASSEMBLE_COMPONENTS<class MATRIX_DSL_ASSIGN_DEFAULTS<class MATRIX_DSL_PARSER<struct MatrixDSL::matrix<int,struct MatrixDSL::structure<struct MatrixDSL::rect<struct MatrixDSL::stat_val<struct MatrixDSL::int_number<int,7>>,struct MatrixDSL::stat_val<struct MatrixDSL::int_number<int,7>>,struct MatrixDSL::unspecified_DSL_feature>,struct MatrixDSL::dense<struct MatrixDSL::unspecified_DSL_feature>,struct MatrixDSL::dyn<struct MatrixDSL::unspecified_DSL_feature>>,struct MatrixDSL::speed<struct MatrixDSL::unspecified_DSL_feature>,struct MatrixDSL::unspecified_DSL_feature,struct MatrixDSL::unspecified_DSL_feature,struct MatrixDSL::unspecified_DSL_feature,struct MatrixDSL::unspecified_DSL_feature>>::DSLConfig>::DSLConfig>>>>>,class MatrixICCL::Matrix<class MatrixICCL::BoundsChecker<class MatrixICCL::ArrFormat<class MatrixICCL::StatExt<struct MatrixDSL::int_number<int,7>,struct MatrixDSL::int_number<int,7>>,class MatrixICCL::Rect<class MatrixICCL::StatExt<struct MatrixDSL::int_number<int,7>,struct MatrixDSL::int_number<int,7>>>,class MatrixICCL::Dyn2DCContainer<class MATRIX_ASSEMBLE_COMPONENTS<class MATRIX_DSL_ASSIGN_DEFAULTS<class MATRIX_DSL_PARSER<struct MatrixDSL::matrix<int,struct MatrixDSL::structure<struct MatrixDSL::rect<struct MatrixDSL::stat_val<struct MatrixDSL::int_number<int,7>>,struct MatrixDSL::stat_val<struct MatrixDSL::int_number<int,7>>,struct MatrixDSL::unspecified_DSL_feature>,struct MatrixDSL::dense<struct MatrixDSL::unspecified_DSL_feature>,struct Ma…

(A + B) * C : misuse of variable produces:

4 Dec 2001

Kestrel 30

Patterns and Errors

4 Dec 2001

Kestrel 31

Pattern Macros: A Full Example

• expansion rules • instance from GoF

• consistency error • syntax check, scope check• type check

4 Dec 2001

Kestrel 32

The Composite Pattern: New Syntax

(COMPOSITE

(ABSTRACT <abs:variable> (FIELDS (<f:variable> <f:exp>)...) (METHODS (<m:variable> <m:exp>)) (ABSTRACT:METHODS <am:variable> ...)) (CONCRETE <con1:variable> (FIELDS (<cf:variable> <cf:exp>) ...) (METHODS (<cm:variable> <cm:exp>) ...)) (COMPOSITE:METHODS (<cm:variable> <cm:exp>) ...))

4 Dec 2001

Kestrel 33

(begin ;; --- ABSTRACT CLASS (define <abs:variable> (class* object% () () (private (<f:variable> <f:exp>) ...) (public (<am:variable> (lambda x (error '<am:variable> "not implemented ...))) ;; --- CONCRETE CLASSES (define <con1:variable> (class* etc. ….. ………………………………………………)))

The Composite Pattern: Elaboration

4 Dec 2001

Kestrel 34

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Kestrel 37

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Kestrel 38

Source Correlation

• to track source information through expansions

• to provide feedback in terms of original source

4 Dec 2001

Kestrel 39

Elaboration Tracking

• to keep track of history of macro transformations

• to help IDE untangle complex interactions between complex pattern embeddings

4 Dec 2001

Kestrel 40

Conclusion

4 Dec 2001

Kestrel 41

Summary

• patterns have a long history, pre-GoF

• patterns should be language constructs for the obvious reasons

• macros (and an expressive base language) can turn most patterns into language constructs

4 Dec 2001

Kestrel 42

Patterns via Macros

• abstract factory, factory, singleton, adapter, bridge, composite, decorator, façade, proxy, chain, command, interpreter, iterator, observer, state, strategy, visitor, template

• from local macros using a base language with higher-order functions and assignment

4 Dec 2001

Kestrel 43

Challenges

• global patterns

• a well-founded theory of patterns

• more experience with pattern in languages with explicit type declarations

4 Dec 2001

Kestrel 44

The End

• Shriram Krishnamurthi

• Daniel Friedman

• Cormac Flanagan

• and PLT