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_searchendExample 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_reservationtrueendendOutput
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: trueUsinginstance_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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