Wednesday, December 10, 2008

The OCaml code again

I'm posting a slight variation of the OCaml code that I think better shows what I like about the ML version.
module MkPerson(O: sig 
                     type xString
                     type xDouble
                     val opConcat : xString -> xString -> xString
                     val opShow : xDouble -> xString
                   end) =
  open O
  type person = Person of (xString * xString * xDouble)
  let display (Person (firstName, lastName, height)) = 
    opConcat firstName (opConcat lastName (opShow height))

module BasicPerson = MkPerson(struct
                                type xString = string
                                type xDouble = float
                                let opConcat = (^)
                                let opShow = string_of_float

open BasicPerson

let _ = 
  let p = Person ("Stefan", "Wehr", 184.0)
  in display p
Note how the open O opens the argument to the MkPerson functor and all the values and types from the argument is in scope in the rest of the module. There's no need to change lots of code in MkPerson.

Similarely, the open BasicPerson makes the operations from that module avaiable, so that Person and display can be used in a simple way.

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