Metaprogramming in Ruby allows developers to write programs that can modify themselves at runtime. This powerful feature can be leveraged to create expressive and flexible domain-specific languages (DSLs).
Defining the Structure
First, let's define the core class of our DSL,TravelSearch
, along with auxiliary classesAir
,Hotel
, andTrain
. These classes will encapsulate the details of each travel component.
classTravelSearchattr_accessor:air,:hotel,:traindefinitialize(&block)instance_eval(&block)enddefair(&block)@air=Air.new(&block)enddefhotel(&block)@hotel=Hotel.new(&block)enddeftrain(&block)@train=Train.new(&block)enddefto_s"Air:#{@air}\nHotel:#{@hotel}\nTrain:#{@train}"endendclassAirattr_accessor:from,:to,:datedefinitialize(&block)instance_eval(&block)enddeffrom(from)@from=fromenddefto(to)@to=toenddefdate(date)@date=dateenddefto_s"from#{@from} to#{@to} on#{@date}"endendclassHotelattr_accessor:city,:date,:nightsdefinitialize(&block)instance_eval(&block)enddefcity(city)@city=cityenddefdate(date)@date=dateenddefnights(nights)@nights=nightsenddefto_s"in#{@city} on#{@date} for#{@nights} nights"endendclassTrainattr_accessor:from,:to,:via,:date,:with_seat_reservationdefinitialize(&block)instance_eval(&block)enddeffrom(from)@from=fromenddefto(to)@to=toenddefvia(via)@via=viaenddefdate(date)@date=dateenddefwith_seat_reservation(with_seat_reservation)@with_seat_reservation=with_seat_reservationenddefto_s"from#{@from} to#{@to} via#{@via} on#{@date} with seat reservation:#{@with_seat_reservation}"endend
Utilizinginstance_eval
for DSL Construction
Theinstance_eval
method is a key component in our DSL. It allows us to evaluate the given block within the context of the current object, effectively changing theself
to the object the method is called on. This is crucial for making the DSL syntax intuitive and clean.
Creating the DSL Entry Point
We'll define a methodsearch_travel
as the entry point for our DSL. This method initializes aTravelSearch
object and evaluates the block within its context.
defsearch_travel(&block)travel_search=TravelSearch.new(&block)putstravel_searchend
Example Usage
Here's an example of how our DSL can be used to define a travel search:
search_traveldoairdofrom"SFO"to"JFK"date"2011-12-25"endhoteldocity"New York"date"2011-12-25"nights3endtraindofrom"Milan"to"Rome"via"Florence"date"2011-12-25"with_seat_reservationtrueendend
Output
Running the above code will produce the following output:
Air: from SFO to JFK on 2011-12-25Hotel: in New York on 2011-12-25 for 3 nightsTrain: from Milan to Rome via Florence on 2011-12-25 with seat reservation: true
Usinginstance_eval
and metaprogramming, we created a flexible and readable DSL for travel searches in Ruby. This approach allows users to define complex data structures with minimal syntax, making the code more expressive and easier to understand.
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