Metaprogramming in Ruby

IntroductionDSL implementation

Summary

Metaprogramming in RubyDomain specific languages

Peter [email protected]

BRUG meeting October 3 2007

Peter Vanbroekhoven [email protected] Metaprogramming in Ruby


Summary

Outline

1 IntroductionMetaprogrammingRuby and Metaprogramming

2 DSL implementationChanging selfMissing constantsMissing methodsImplementing custom operators



Summary

MetaprogrammingRuby and Metaprogramming

Outline





Summary


What Is Metaprogramming?

Programmingyour

programming



Summary


What Is Metaprogramming?

Metaprogrammingis a

good thing



Summary


The Deal With Metaprogramming

class ActiveRecord : : Basedef r e a d _ a t t r i b u t e ( attr_name )

@at t r ibu tes [ attr_name ]end

endclass MicrosoftCEO < ActiveRecord : : Baseend

b i l l = MicrosoftCEO . newmy_pocket << b i l l . r e a d _ a t t r i b u t e ( "money" )



Summary



class MicrosoftCEO < ActiveRecord : : Basedef money

@at t r ibu tes [ "money" ]end

end

b i l l = MicrosoftCEO . newmy_pocket << b i l l . money



Summary



class MicrosoftCEO < ActiveRecord : : Basedef money

@at t r ibu tes [ "money" ]enddef house

@at t r ibu tes [ " house " ]enddef car

@at t r ibu tes [ " car " ]end

end

b i l l = MicrosoftCEO . newmy_pocket << b i l l . money << b i l l . house << b i l l . car



Summary



class MicrosoftCEO < ActiveRecord : : Base[ "money" , " house " , " car " ] . each do | f |

define_method ( f ) { @at t r ibu tes [ f ] }end

end

b i l l = MicrosoftCEO . newmy_pocket << b i l l . money << b i l l . house << b i l l . car



Summary


Examples of metaprogramming

Java compilerRuby interpreterFirefox browserRuby on Rails



Summary


Domain specific languages

def i s _ s t u f f _ c h a r ( c )( c >= ?a && c <= ?z ) | | ( c >= ?A && c <= ?Z) | |( c >= ?0 && c <= ?9) | | c == ?_

enddef f i n d _ s t u f f ( s t r )

r e s u l t = [ ] ; i = 0 ; while i < s t r . l eng ths t a r t = i ; i += 1 while i s _ s t u f f _ c h a r ( s t r [ i ] )i f s t a r t != i && s t r [ i ] == ’@’

a t = i ; i += 1i += 1 while i s _ s t u f f _ c h a r ( s t r [ i ] )r e s u l t << s t r [ s t a r t . . . i ] i f at != i

end ; i += 1 ; endr e s u l t ; end



Summary



def f i nd_ema i l s ( s t r )s t r . scan ( / \w+@\w+ / )

end



Summary



musical notes guitar chords

DSLsare

everywhere

traffic signs Ruby



Summary



musical notes

''PPPPPPPPPPPP

guitar chords

DSLsare

everywhere

traffic signs Ruby



Summary



musical notes

''PPPPPPPPPPPP guitar chords

wwoooooooooooo

DSLsare

everywhere

traffic signs Ruby



Summary



musical notes


wwoooooooooooo

DSLsare

everywhere

traffic signs

77nnnnnnnnnnnn

Ruby



Summary



musical notes


wwoooooooooooo

DSLsare

everywhere

traffic signs

77nnnnnnnnnnnnRuby

ggOOOOOOOOOOOO



Summary


Outline





Summary


Metaprogramming in Java

public Plug in c rea te_p lug in ( S t r i n g name,S t r i n g [ ] keys , S t r i n g [ ] values )

throws . . . {Class c = Class . forName (name ) ;Class s = Class . forName ( " [ L java . lang . S t r i n g ; " ) ;Const ruc tor co = c . ge tCons t ruc to r ( s , s ) ;return ( P lug in ) co . newInstance ( keys , values ) ;

}



Summary


Metaprogramming in Ruby

def c rea te_p lug in (name, keys , values )Object . const_get (name ) . new( keys , values )

end



Summary


Comparison

Metaprogramming in Rubyis

easy



Summary


Comparison

Metaprogramming in Javais

hard



Summary


DSLs in Ruby

f i r s t 1 , f i r s t 2 , l a s t = ’ pe ter ’ , ’ glenn ’ , ’ v\% ’banks = connect ion . execute ( "

SELECT b . nameFROM customer AS c , bank AS b , account AS aWHERE ( c . f i r s tname = ’#{first1} ’ OR

c . f i r s tname = ’#{first2} ’ ) ANDc . lastname LIKE ’#{last} ’ ANDa . owner = c . i d ANDa . bank = b . i d

" )



Summary


DSLs in Ruby : ActiveRecord

Bank . f i n d (: a l l ,: i nc lude => { : accounts => : customer } ,: cond i t i ons => [

" ( customers . f i r s t_name = ? ORcustomers . f i r s t_name = ?) AND

customers . last_name LIKE ? " ," f i r s t 1 " , " f i r s t 2 " , " l a s t " ]

) . map { | b | b . name }



Summary


DSLs in Ruby : my own DSL

f i r s t 1 , f i r s t 2 , l a s t = ’ pe ter ’ , ’ glenn ’ , / \ Av /banks = query do

from : c => Customer , : b => Bank , : a => Accounts e l e c t b . namewhere ( ( c . f i r s tname == f i r s t 1 ) |

( c . f i r s tname == f i r s t 2 ) ) &( c . lastname =~ l a s t ) &( a . owner == c ) &( a . bank == b )

end



Summary




from : a => Accounts e l e c t a . bank . namewhere ( ( a . owner . f i r s tname == f i r s t 1 ) |

( a . owner . f i r s tname == f i r s t 2 ) ) &( a . owner . lastname =~ l a s t )

end



Summary




from : a => Account

c = a . owner

s e l e c t a . bank . namewhere ( ( c . f i r s tname == f i r s t 1 ) |

( c . f i r s tname == f i r s t 2 ) ) &( c . lastname =~ l a s t )

end



Summary


DSLs in Ruby

easy to build familiar syntax

A Ruby DSLis just

plain Ruby

abstraction



Summary


DSLs in Ruby

easy to build

''OOOOOOOOOOO

familiar syntax

A Ruby DSLis just

plain Ruby

abstraction



Summary


DSLs in Ruby

easy to build

''OOOOOOOOOOO familiar syntax

wwnnnnnnnnnnnn

A Ruby DSLis just

plain Ruby

abstraction



Summary


DSLs in Ruby

easy to build

''OOOOOOOOOOO familiar syntax

wwnnnnnnnnnnnn

A Ruby DSLis just

plain Ruby

abstraction

OO



Summary


What I’ll Teach You




end



Summary

Changing selfMissing constantsMissing methodsImplementing custom operators

Outline





Summary


What is self?

instancevariables

dotlessmethods

selfis the

current object

classvariables

changed bymethod calls,class defs



Summary


What is self?

instancevariables

''OOOOOOOOOOO

dotlessmethods

selfis the

current object

classvariables




Summary


What is self?

instancevariables

''OOOOOOOOOOO

dotlessmethods

vvnnnnnnnnnnn

selfis the

current object

classvariables




Summary


What is self?

instancevariables

''OOOOOOOOOOO

dotlessmethods

vvnnnnnnnnnnn

selfis the

current object

classvariables

88qqqqqqqqqqqq




Summary


What is self?

instancevariables

''OOOOOOOOOOO

dotlessmethods

vvnnnnnnnnnnn

selfis the

current object

classvariables

88qqqqqqqqqqqq changed bymethod calls,class defs

ggOOOOOOOOOOO



Summary


What is self?

banks = query do. . .

end



Summary


What is self?

banks = se l f . query do. . .

end



Summary


What is self?

# Top l e v e lp se l f # => mainp se l f . class # => Object



Summary


What is self?

banks = se l f . query do. . .

end



Summary


The Kernel module

puts raise

Kernel

require eval



Summary


The Kernel module

puts

''OOOOOOOOOOO

raise

Kernel

require eval



Summary


The Kernel module

puts

''OOOOOOOOOOO raise

xxqqqqqqqqqqq

Kernel

require eval



Summary


The Kernel module

puts

''OOOOOOOOOOO raise

xxqqqqqqqqqqq

Kernelrequire

77ooooooooooo

eval



Summary


The Kernel module

puts

''OOOOOOOOOOO raise

xxqqqqqqqqqqq

Kernelrequire

77oooooooooooeval

ffMMMMMMMMMMM



Summary


The query method

module Kerneldef query

yie ld # Ca l l the blockend

end



Summary


Changing self

banks = query dofrom : c => Customer , : b => Bank , : a => Account. . .

end



Summary


Changing self

banks = query dose l f . from : c => Customer , . . .. . .

end



Summary


Changing self

banks = query dop se l f # => main

end



Summary


Changing self

banks = query dose l f . from : c => Customer , . . .. . .

end



Summary


Changing self

Sometimeswe want tochange self



Summary


Changing self

class Queryendmodule Kernel

def query (& b lk )Query . new . ins tance_eva l (& b lk )

endend



Summary


Changing self

banks = query dop se l f # => #<Query :0 xb7cc4adc>

end



Summary


Changing self

instance_evalchanges self

for the duration of a block



Summary


Changing self

class Querydef i n i t i a l i z e

@from = { }enddef from (map)

@from = mapend

end



Summary


Changing self

banks = query dofrom : c => Customer , : b => Bank , : a => Accountp se l f # => #<Query :0 xb7cc4adc# @from = { : b=>Bank , : a=>Account , : c=>Customer } >

end



Summary


Outline





Summary


Lazy class loading

Railscan do

lazy class loading



Summary


Lazy class loading

test.rb

# requ i re ’ brug ’BRUG. new( Time . now)

brug.rb

class BRUGdef i n i t i a l i z e ( t ime )

@time = t imeend

end



Summary


Constant lookup

BRUG.new

n = Module.nesting + [Object]

find BRUG in n

return BRUG n [0]. const_missing(:BRUG)



Summary


Constant lookup

BRUG.new

��n = Module.nesting + [Object]

find BRUG in n




Summary


Constant lookup

BRUG.new


��find BRUG in n




Summary


Constant lookup

BRUG.new



found

zzuuuuuuuuuuuuuuuuuuuu

return BRUG

n [0]. const_missing(:BRUG)



Summary


Constant lookup

BRUG.new



found

zzuuuuuuuuuuuuuuuuuuuu

not found

%%KKKKKKKKKKKKKKKKKKKKK




Summary


Module.nesting

class Amodule B

def mModule . nes t ing # => [A : : B, A ]

endModule . nes t ing # => [A : : B, A ]

endendModule . nes t ing # => [ ]def A.m

Module . nes t ing # => [ ]end



Summary


Lazy class loading

banks = query dofrom : c => Customer , : b => Bank , : a => Accountp Module . nes t ing + [ Object ] # => [ Object ]. . .

end



Summary


Lazy loading

def Object . const_missing ( c id )f i l e = f i n d _ f i l e ( c id )requ i re f i l econst_get ( c id )

end



Summary


Lazy class loading

def Object . const_missing ( c id )f i l e = f i n d _ f i l e ( c id )requ i re f i l ei f const_def ined? c id

const_get ( c id )else

# c a l l o ld const_missingend

end



Summary


Lazy class loading

def Object . const_missing ( c id )f i l e = f i n d _ f i l e ( c id )requ i re f i l ei f const_def ined? c id

const_get ( c id )else

# Class < Modulesuper

endend



Summary


Lazy class loading

Lazy class loadingis

freakingly easy



Summary


Outline





Summary


Handling b.name

banks = query do. . .s e l e c t b . name. . .

end



Summary


Handling b.name

banks = query do. . .s e l e c t se l f . b . name. . .

end



Summary


Handling b.name

class Querydef b

. . .end

end

banks = query do. . .s e l e c t se l f . b . name. . .

end



Summary


Handling b.name

Can we

avoidmass method definition

?



Summary


Missing methods

self .b.name

self .b.name self .method_missing(:b)



Summary


Missing methods

self .b.name

b found

{{wwwwwwwwwwwwwwwwwww

self .b.name

self .method_missing(:b)



Summary


Missing methods

self .b.name

b found

{{wwwwwwwwwwwwwwwwwww

b not found

$$JJJJJJJJJJJJJJJJJJJJ

self .b.name self .method_missing(:b)



Summary


Implementing Query#method_missing

banks = query do. . .p se l f # => #<Query :0 xb7cf5dd4 >s e l e c t se l f . b . name. . .

end



Summary



class Querydef method_missing ( mid , ∗args )

i f args . leng th == 0 && @from [m]Var iab leExpress ion . new(m)

elsesuper

endend

end



Summary



class Var iab leExpress ion < Expression

a t t r _ r e a d e r : varname

def i n i t i a l i z e ( varname )@varname = varname

end

end



Summary


Alternative implementation

class Querya l ias old_mm method_missingdef method_missing (m, ∗args )

i f args . leng th == 0 && @from [m]Var iab leExpress ion . new(m)

elseold_mm (m, ∗args )

endend

end



Summary


Implementing Expression#method_missing

banks = query do. . .# s e l e c t bs e l e c t b . name. . .

end



Summary



class Expression

def method_missing (m, ∗args )i f args . leng th == 0

Fie ldExpress ion . new( sel f , m)else

superend

end

end



Summary



class Fie ldExpress ion < Expressiondef i n i t i a l i z e ( expression , f i e l d )

@expression , @f ie ld = expression , f i e l dend

end



Summary


Summary

YES,we can

avoidmass method definition



Summary


Outline





Summary


Oerator overloading

banks = query do. . .where ( ( c . f i r s tname == f i r s t 1 ) |


end



Summary


Operator overloading

Operatorsare just

methods



Summary


Operator overloading

banks = query do. . .where ( ( c . f i r s tname .==( f i r s t 1 ) . | (

c . f i r s tname .==( f i r s t 2 ) ) . & (c . lastname .=~( l a s t ) . & (a . owner == c ) . & ( a . bank .==( b ) )

end



Summary


Implementing operator overloading

class Expressiondef ==( o ther )

BinaryOpExpression . new( sel f , other , ’ = ’ )enddef &( other )

BinaryOpExpression . new( sel f , other , ’AND ’ )end. . .

end



Summary


Implementing operator overloading

class BinaryOpExpression < Expression

def i n i t i a l i z e ( expr1 , expr2 , op )@expr1 , @expr2 , @op = expr1 , expr2 , op

end

end



Summary


Limitations operator overloading

&&, ||, !are

NOTmethods



Summary


Summary

but ...



Summary


Limitations operator overloading

Operator overloadingis

child’s play



Summary


Wrapping up the example

banks = query do. . .where . . .

end



Summary



class Query

def where ( cond )@where = cond

end

end



Summary



class Query

def t o_sq l"SELECT #{ r e t u r n _ l i s t } " \"FROM #{ t a b l e _ l i s t } " \"WHERE #{ cond i t i on_expr } "

end

end



Summary



class Querydef t a b l e _ l i s t

@from .map { | v , t | " #{ t } AS #{ v } " } . j o i n ( " , " )end

def r e t u r n _ l i s t@select .map { | r | r . t o_sq l } . j o i n ( " , " )

end

def cond i t i on_expr@where . to_sq l

endend



Summary






end



Summary

Summary

Ruby has very natural metaprogramming facilitiesRuby allows building DSLs in pure RubyMetaporgramming made easy



Summary

Further reading

why’s (poignant) guide to Rubyhttp://poignantguide.net/ruby/http://poignantguide.net/ruby/chapter-6.html

Ruby for Railshttp://www.manning.com/black/

Programming Rubyhttp://www.rubycentral.com/pickaxe/http://www.pragmaticprogrammer.com/title/ruby/



Summary

REAL Summary

Ruby rocks!!!


Metaprogramming in Ruby

Technology

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