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Domain Specific Languages
- a user view and an
implementation view
does expressiveness imply a compromise on the
underlying domain model ?
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Debasish Ghosh
@debasishg on twitter
Code @ http://github.com/debasishg
Blog @ Ruminations of a Programmerhttp://debasishg.blogspot.com
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Open Source Footprints
• Committer in Akka (http://akka.io)
• Owner/Founder of :– sjson (JSON serialization library for Scala objects –
http://github.com/debasishg/sjson)
– scala-redis (Redis client for Scala – http://github.com/debasishg/scala-redis)
– scouchdb (Scala driver for Couchdb – http://github.com/debasishg/scouchdb)
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• model-view architecture of a DSL
• how abstractions shape a DSL structure
• choosing proper abstractions for compositionality
• using proper language idioms
• composing DSLs
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•model-view architecture of a DSL
• how abstractions shape a DSL structure
• choosing proper abstractions for compositionality
• using proper language idioms
• composing DSLs
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ViewOr
Syntax
new_trade 'T-12435' for account 'acc-123' to buy 100 shares of 'IBM', at UnitPrice = 100, Principal = 12000, Tax = 500
DSL
Model…
Semantic modelof the domain
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view / syntax
is derived from
model
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view / syntax
is a veneer of abstraction over
model
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view / syntax
is decoupled thru an interpreter from
model
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view / syntax
can be multiple over the same
model
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# create instrumentinstrument = Instrument.new('Google')instrument.quantity = 100
#create tradeTrade.new('T-12435', 'acc-123', :buy, instrument, 'unitprice' => 200, 'principal' => 120000, 'tax' => 5000)
idiomatic Ruby API
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new_trade 'T-12435' for account 'acc-123' to buy 100 shares of 'IBM',
at UnitPrice = 100, Principal = 12000, Tax = 500
add domain syntax
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new_trade 'T-12435' for account 'acc-123' to buy 100 shares of 'IBM',
at UnitPrice = 100, Principal = 12000, Tax = 500
add domain syntax
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domain syntax
idiomatic Ruby model
interpreter
isolation layer between the model and the view of the DSL
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def parse(dsl_string) dsl = dsl_string.clone dsl.gsub!(/=/, '=>') dsl.sub!(/and /, '') dsl.sub!(/at /, '') dsl.sub!(/for account /, ',') dsl.sub!(/to buy /, ', :buy, ') dsl.sub!(/(\d+) shares of ('.*?')/, '\1.shares.of(\2)') puts dsl dslend
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Not all transformations can be done using simple regular expression
based string processing
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• model-view architecture of a DSL
• how abstractions shape a DSL structure
• choosing proper abstractions for compositionality
• using proper language idioms
• composing DSLs
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… …
abstractions of the core domainmodel
layers of abstractions
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… …abstractions of the core domainmodel
abstractions thatglue modelelements
layers of abstractions
combinators, façade objects, bridges & adapters
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… …abstractions of the core domainmodel
abstractions thatglue modelelements
layers of abstractions
combinators, façade objects, bridges & adapters
domain specific embedded language
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… …abstractions of the core domainmodel
abstractions thatglue modelelements
layers of abstractions
combinators, façade objects, bridges & adapters
domain specific embedded language
mod
el
vie
w
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layers of abstractions ..
• keep the underlying domain model clean• ensure separation of concerns between
the model and the view of the DSL• you can also implement the glue layer
using a different programming language (polyglot programming)
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withAccount(trade) { account => { settle(trade using account.getClient .getStandingRules .filter(_.account == account) .first) andThen journalize }}
DSL in Scala over a Java domain model
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withAccount(trade) { account => { settle(trade using account.getClient .getStandingRules .filter(_.account == account) .first) andThen journalize }}
DSL in Scala over a Java domain model
Java abstractions
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withAccount(trade) { account => { settle(trade using account.getClient .getStandingRules .filter(_.account == account) .first) andThen journalize }}
DSL in Scala over a Java domain model
Java abstractions
Wired throughScala control structures
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We need abstractions that generalize over computations ..
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Our domain – stocks, trades, bulls, bears ..
• Security trade– Exchange of securities and currencies (also
known as Instruments)– In the stock exchange (market)– On a specified "trade date"– Between counter parties– In defined quantities– At market rates– Resulting in a net cash value for the seller– On the "value date"
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Our domain – stocks, trades, bulls, bears ..
• Security trade– Exchange of securities and currencies (also
known as Instruments)– In the stock exchange (market)– On a specified "trade date"– Between counter parties' accounts– In defined quantities– At market rates– Resulting in a net cash value for the seller– On the "value date"
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// The Trade modelcase class Trade( account: Account, instrument: Instrument, referenceNo: String, market: Market, unitPrice: BigDecimal, quantity: BigDecimal, tradeDate: Date = Calendar.getInstance.getTime, valueDate: Option[Date] = None, taxFees: Option[List[(TaxFeeId, BigDecimal)]] = None, netAmount: Option[BigDecimal] = None)
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// various tax/fees to be paid when u do a tradesealed trait TaxFeeIdcase object TradeTax extends TaxFeeIdcase object Commission extends TaxFeeIdcase object VAT extends TaxFeeIdcase object Surcharge extends TaxFeeId
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// computes value date of a tradeval valueDateProcessor: Trade => Trade = //.. impl
// computes tax/fee of a tradeval taxFeeProcessor: Trade => Trade = //.. Impl
// computes the net cash value of a tradeval netAmountProcessor: Trade => Trade = //.. impl
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val trades = //.. list of security trades
// get a list of value dates for all trades val valueDates = trades map (_.valueDate)
// enrich a trade passing it thru multiple processors val richTrade = (valueDateProcessor map taxFeeProcessor map netAmountProcessor) apply trade
// pass a trade thru one specific processorval t = some(trade) map valueDateProcessor
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val trades = //.. list of security trades
// get a list of value dates for all trades
val valueDates = trades map (_.valueDate)
// enrich a trade passing it thru multiple processors
val richTrade = (valueDateProcessor map taxFeeProcessor map netAmountProcessor) apply trade
// pass a trade thru one specific processor
val t = some(trade) map valueDateProcessor
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val trades = //.. list of security trades
// get a list of value dates for all trades
val valueDates = trades map (_.valueDate)
// enrich a trade passing it thru multiple processors
val richTrade = (valueDateProcessor map taxFeeProcessor map netAmountProcessor) apply trade
// pass a trade thru one specific processor
val t = some(trade) map valueDateProcessor
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val trades = //.. list of security trades
// map across the list functorval valueDates = trades map (_.valueDate)
// map across the Function1 functor val richTrade = (valueDateProcessor map taxFeeProcessor map netAmountProcessor) apply trade
// map across an Option functorval t = some(trade) map valueDateProcessor
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a map is ..
a combinator
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a map ..
pimps abstractions to give us an ad hoc polymorphism implementation on top of OO
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a map is ..
fundamentally powered by type classes
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the idea is to generalize abstractions ..
.. and by generalizing map over functors we get a combinator that can glue a large set of our domainabstractions
.. and this gives a feeling of uniformity in the DSL syntax
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semantic model+
combinators=
DSL
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• model-view architecture of a DSL
• how abstractions shape a DSL structure
• choosing proper abstractions for compositionality
• using proper language idioms
• composing DSLs
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a DSL ..
• evolves from the model non-invasively• using combinators• that compose model elements• interprets them into computations of the domain
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so, a domain model also has to be ..
designed for compositionality
Image source: Mathematical art of M. C. Escher (http://im-possible.info/english/articles/escher_math/escher_math.html)
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domain rule ..
enrichment of trade is done in steps:
1. get the tax/fee ids for a trade2. calculate tax/fee values3. enrich trade with net cash amount
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the DSL ..
val enrich = for { // get the tax/fee ids for a trade taxFeeIds <- forTrade
// calculate tax fee values taxFeeValues <- taxFeeCalculate
// enrich trade with net cash amount netAmount <- enrichTradeWith}yield((taxFeeIds map taxFeeValues) map netAmount)
// enriching a tradetrade map enrich should equal(…)
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the DSL ..
val enrich = for { // get the tax/fee ids for a trade taxFeeIds <- forTrade
// calculate tax fee values taxFeeValues <- taxFeeCalculate
// enrich trade with net cash amount netAmount <- enrichTradeWith}yield((taxFeeIds map taxFeeValues) map netAmount)
// enriching a tradetrade map enrich should equal(…)
Model elementsthat can be composedusing the for-comprehension
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reiterating ..
a combinator acts ..as a glue combining model elements
But ..
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there is a prerequisite ..
model elements should be composable
we call this the compositionality of the model
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Q: How to design your model
so that it's compositional ?
A: choose the right abstractions ..
depending on the programming paradigm (FP, OO ..) and the implementation
language used
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domain rule ..
net cash value calculation of trade:
1. use strategy for specific markets, if available2. otherwise use default strategy3. we can supply specific strategy inline as well
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the DSL ..
// type alias for domain vocabularytype NetAmount = BigDecimaltype CashValueCalculationStrategy = PartialFunction[Market, Trade => NetAmount]
// declarative business logicval cashValueComputation: Trade => NetAmount = { trade => (forHongKong orElse forSingapore orElse forDefault)(trade.market)(trade)}
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the DSL ..
val cashValue: Trade => CashValueCalculationStrategy => NetAmount = { trade => pf => if (pf.isDefinedAt(trade.market)) pf(trade.market)(trade) else cashValueComputation(trade)}
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PartialFunction is ..the secret sauce
• all functions for specific and default computation
return PartialFunction
• that offer combinators for chaining of abstractions ..
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val forHongKong: CashValueCalculationStrategy = { case HongKong => { trade => //.. }}
val forSingapore: CashValueCalculationStrategy = { case Singapore => { trade => //.. }}
val forDefault: CashValueCalculationStrategy = { case _ => { trade => //.. }}
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• model-view architecture of a DSL
• how abstractions shape a DSL structure
• choosing proper abstractions for compositionality
• using proper language idioms
• composing DSLs
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choice of abstractions depends
on the programming language chosen for implementation of your DSL
always keep an eye on the paradigms that yourimplementation language supports and play to
the idioms of the language
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domain rule ..
A trade needs to be enriched with a set of tax and feesbefore we calculate its net cash value
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the Java way ..
// use the decorator patternnew CommissionDecorator( new TaxFeeDecorator(new Trade(..)));
public class Trade { //..
public class TaxFeeDecorator extends Trade { private Trade trade; //..}public class CommissionDecorator extends Trade { private Trade trade; //..}
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the Java way ..
• decorators and the decoratee share a common interface
• statically coupled through inheritance• the decorator usage syntax is outside-in
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the Ruby way ..
## decorator patternTrade.new(..).with TaxFee, Commission
class Trade attr_accessor .. def initialize .. def with(*args) args.inject(self) {|memo, val| memo.extend val} end def value @principal endend
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the Ruby way ..
## mixinsmodule TaxFee ## calculate taxfee def value super + .. endend
## mixinsmodule Commission ## calculate commission def value super + .. endend
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the Ruby way ..
• more dynamic compared to Java• uses Ruby idioms of runtime meta-programming• decorators don’t statically extend the core abstraction
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if we were to do the same in Clojure ..
## decorator pattern(with-tax-fee trade (with-values :tax 12) (with-values :commission 23))
(defmacro with-tax-fee "wrap a function in one or more decorators" [func & decorators] `(redef ~func (-> ~func ~@decorators)))
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the Clojure way ..
• more functional – uses HOF• function threading makes the decorator
implementation almost trivial• control structure uses macros, which are compile
time meta-programming artifacts (unlike Ruby) – no run time overhead
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• model-view architecture of a DSL
• how abstractions shape a DSL structure
• choosing proper abstractions for compositionality
• using proper language idioms
• composing DSLs
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if you thought DSLs grow through composition of abstractions ..
.. then the final DSL is the whole formed from the composition of its parts ..
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… and if the final DSL is also an abstraction ..
.. there's no reason we can compose it with another abstraction (aka another DSL) to form a bigger whole ..
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trait TradeDsl { type T <: Trade def enrich: PartialFunction[T, T]
//.. other methods}
constrained abstract type
abstract method
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trait FixedIncomeTradeDsl extends TradeDsl { type T = FixedIncomeTrade
import FixedIncomeTradingService._ override def enrich: PartialFunction[T, T] = { case t => t.cashValue = cashValue(t) t.taxes = taxes(t) t.accruedInterest = accruedInterest(t) t }}
object FixedIncomeTradeDsl extends FixedIncomeTradeDsl
concrete type for fixed income trade
concrete definition forenrichment of fixed income trade
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trait EquityTradeDsl extends TradeDsl { type T = EquityTrade
import EquityTradingService._ override def enrich: PartialFunction[T, T] = { case t => t.cashValue = cashValue(t) t.taxes = taxes(t) t }}
object EquityTradeDsl extends EquityTradeDsl
concrete type for equity trade
concrete definition forenrichment of equity trade
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.. and now a new market rule comes up that needs to be applied to all types of trades
.. you don’t want to affect the core logic, since the rule is a temporary and promotional one and is likely to be discontinued after a few days ..
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.. design the DSL for the new rule in such a way that you can plug in the semantics of all types of trade within it ..
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and now the rule itself ..
“Any trade on the New York Stock Exchange of principal value > 10,000 USD must have a discount of 10% of the principal on the net cash value.”
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trait MarketRuleDsl extends TradeDsl { val semantics: TradeDsl type T = semantics.T override def enrich: PartialFunction[T, T] = { case t => val tr = semantics.enrich(t) tr match { case x if x.market == NYSE && x.principal > 1000 => tr.cashValue = tr.cashValue - tr.principal * 0.1 tr case x => x } }}
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trait MarketRuleDsl extends TradeDsl { val semantics: TradeDsl type T = semantics.T override def enrich: PartialFunction[T, T] = { case t => val tr = semantics.enrich(t) tr match { case x if x.market == NYSE && x.principal > 1000 => tr.cashValue = tr.cashValue - tr.principal * 0.1 tr case x => x } }}
polymorphic embedding
new business rule that actson top of the enriched trade
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object EquityTradeMarketRuleDsl extends MarketRuleDsl { val semantics = EquityTradeDsl}
object FixedIncomeTradeMarketRuleDsl extends MarketRuleDsl { val semantics = FixedIncomeTradeDsl}
pluggable composition of DSLs
plug-in the concrete semantics
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import FixedIncomeTradeMarketRuleDsl._
withTrade( 200 discount_bonds IBM for_client NOMURA on NYSE at 72.ccy(USD)) {trade => Mailer(user) mail trade Logger log trade } cashValue
a sneak peek at the
fully composed DSL ..
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Summary
• A DSL has a syntax (for the user) and a model (for the implementer)
• The syntax is the view designed on top of the model
• The model needs to be clean and extensible to support seamless development of domain syntax
• The model needs to be compositional so that various model elements can be glued together using combinators to make an expressive syntax
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