Sharedexpensez

eDSL example using a State monad from Scalaz

5 years after

eDSL using State monad from Scalaz

This is an example converted to Scala from Haskell. The original example:

http://lpenz.github.com/articles/hedsl-sharedexpenses/index.html

An example

val dexter = Person("Dexter")
val angel  = Person("Angel")
val debra  = Person("Debra")
val harry  = Person("Harry")

val trip = sharedexpenses ( for {
  _ <- dexter spent 5300
  _ <- angel  spent 2700
  _ <- debra  spent  800
  _ <- harry  spent 1900
  _ <- debra  spent 1700
  _ <- angel  spent 2200
  _ <- dexter gave  (harry, 2000)
  x <- angel  gave  (debra, 3200)
} yield x)

scala> println(solve(trip).mkString("\n"))

Debra pays 4350 to Angel
Harry pays 3650 to Dexter
Harry pays 100 to Angel

Conclusion

Scala's for-comprehension is not quite as nice as Haskell's do-notation in this example. There's some syntactic noise coming from required binding '_ <- ' and from required value yielding. It would be perhaps possible to remove those if Scala would add some sugar to its for-comprehension:

  • Do not require variable binding for M[Unit]
  • yield the value of a last expression if yield is missing

After that the example would look as:

val dexter = Person("Dexter")
val angel  = Person("Angel")
val debra  = Person("Debra")
val harry  = Person("Harry")

val trip = sharedexpenses ( for {
  dexter spent 5300
  angel  spent 2700
  debra  spent  800
  harry  spent 1900
  debra  spent 1700
  angel  spent 2200
  dexter gave  (harry, 2000)
  angel  gave  (debra, 3200)
})

Kudos for the original poster for a nice example!