ML modules Daniel Jackson MIT Lab for Computer Science 6898 Advanced Topics in Software Design March 20 2002 topics for today elements of ML module language structs modules export types and values signatures types for modules functors functions from modules to modules signature ascription controlling client s view of a module functorization making dependences explicit 2 set implementation def and use module SetImpl struct type a t a list let empty let add s e e s let member s e List mem e s end let s SetImpl empty SetImpl add s 3 3 one possible type for the module module SetImplC ManifestSet struct type a t a list let empty let add s e e s let member s e List mem e s end module type ManifestSet sig type a t a list val empty unit a t val add a t a a t val member a t a bool end 4 another type for the same module module SetImplA OpaqueSet struct type a t a list let empty let add s e e s let member s e List mem e s end module type OpaqueSet sig type a t val empty unit a t val add a t a a t val member a t a bool end 5 controlling access let s SetImplC empty SetImplC add s 3 4 s let s SetImplA empty SetImplA add s 3 4 s type error 6 extending a module module SetWithUnion struct include SetImpl let union s1 s2 List append s1 s2 end 7 substructure suppose we want a set of strings exploiting ordering module OrderedString struct type t string let lt a b a b end 8 module OrderedStringSet struct module Os OrderedString type t Os t let empty let rec add s e match s with e x xs if Os lt e x then e s else x add xs e let rec member s e match s with false x xs if Os lt x e then false else member xs e end does this satisfy the signature OpaqueSet 9 making it generic module type Ordered sig type t val lt t t bool end 10 a functor module OrderedSetImpl functor Elt Ordered struct type element Elt t type set Elt t list let empty let rec add s e match s with e x xs if Elt lt e x then e s else x add xs e let rec member s e match s with false x xs if Elt lt x e then false else member xs e end 11 a small design problem design a program takes names phone numbers as input saves and restores from a file does lookup of number given name 12 a generic parseable type module type PARSEABLE sig type t val parse string t val unparse t string end use parse unparse for unmarshal marshal too 13 a file module type module type FILEFUN functor K PARSEABLE functor V PARSEABLE sig type keytype K t type valuetype V t type filetype val empty unit filetype val read filetype keytype valuetype Hashtbl t unit val write filetype keytype valuetype Hashtbl t unit end 14 a file implementation module File FILEFUN functor K PARSEABLE functor V PARSEABLE struct type keytype K t type valuetype V t type filetype string string list ref let empty ref let read file tbl let insert p Hashtbl add tbl K parse fst p V parse snd p in List iter insert file let write file tbl let cons k v l K unparse k V unparse v l in file Hashtbl fold cons tbl end 15 a generic file backed mapper module Mapper functor K PARSEABLE functor V PARSEABLE struct module KVF File K V type keytype K t type valuetype V t let file KVF empty let tbl Hashtbl create 10 let save KVF write file tbl let restore KVF read file tbl let put k v Hashtbl add tbl k v let get k Hashtbl find tbl k let remove k Hashtbl remove tbl k let has k Hashtbl mem tbl k end 16 names phone numbers module Name PARSEABLE struct type t string let parse x x let unparse x x end module PhoneNumber PARSEABLE struct type t areacode string rest string let parse s areacode String sub s 0 3 rest String sub s 4 7 let unparse n String concat n areacode n rest end 17 a phonebook implementation module PB struct module M Mapper Name PhoneNumber include M let enter name num M put Name parse name Num parse num let lookup name let n Name parse name in if M has n then PhoneNumber unparse M get n else missing end 18 using the phonebook PB enter home 617 9644620 unit PB lookup home string 617 9644620 PB save PB enter office 617 2588471 PB lookup office string 617 2588471 PB enter home 617 9999999 PB restore PB lookup home string 617 9644620 19 fully functorizing 1 module type MAPPERFUN functor K Parseable functor V Parseable sig type keytype K t type valuetype V t val save unit unit val restore unit unit val put keytype valuetype unit val get keytype valuetype val has keytype bool val remove keytype unit end 20 fully functorizing 2 module PBFun functor Name PARSEABLE functor Num PARSEABLE functor MF MAPPERFUN struct module M MF Name Num include M let enter name num M put n Name parse name Num parse num let lookup name let n Name parse name in if M has n then Num unparse M get n else missing end 21 putting it all together module MyPB PBFun Name PhoneNumber Mapper MyPB enter home 617 9644620 MyPB lookup home 22 notes functorizing eliminates global references makes dependences explicit but parameter proliferation can be cumbersome Mapper is a singleton probably a bad design object oriented solution would require a separate factory class using serialization avoids this but relies on extra linguistic mechanism and doesn t give readable file 23 will this work module MarriageRegFun functor Man PARSEABLE functor Woman PARSEABLE functor MF MAPPERFUN struct module M MF Man Woman include M let enter a b let a Man parse a and b Woman parse b in M put a b M put b a let lookup name let n Man parse name in if M has n then Woman unparse M get n else missing end 24 sharing constraints module MarriageRegFun functor Man PARSEABLE functor Woman PARSEABLE with type t Man t functor MF MAPPERFUN struct module M MF Man Woman include M let enter a b let a Man parse a and b Woman parse b in M put a b M put b a let lookup name let n Man parse name in if M has n then Woman unparse M get n else missing end 25 discussion what does ML offer over Java why aren t sharing constraints a big deal in Java came up in discussion can a Caml module have two components with same name apparently yes with matching or different types in signature and structure one seems to shadow the other why 26