I am attempting to learn oCaml and am having an issue as to why the below program is invalid.
class myClass2 =
object
method doSomething = Printf.printf "%s\n" "Doing something"
end;;
class myClass foo =
object
val dMember = foo
method doIt = dMember#doSomething
end;;
let mc2 = new myClass2;;
let mc = new myClass mc2;;
mc#doIt;;
The error I receive when trying to compile the program is:
File "sample.ml", line 6, characters 5-84:
Some type variables are unbound in this type:
class myClass :
(< doSomething : 'b; .. > as 'a) ->
object val dMember : 'a method doIt : 'b end
The method doIt has type 'a where 'a is unbound
I am particularly interested as to why:
val dMember = foo
method doIt = dMember#doSomething
is invalid. Any (and I mean any) help is appreciated.
OCaml objects can't have free type variables in their signatures. Since the type of the argument foo is not fully specified, you need to parameterize myClass by the free variables in the type of foo.
class ['a] myClass foo =
object
val dMember = foo
method doIt : 'a = dMember#doSomething
end;;
This definition has the type
class ['a] myClass :
(< doSomething : 'a; .. > as 'b) ->
object val dMember : 'b method doIt : 'a end
This is similar to normal parameterized dataypes, e.g., 'a tree (the brackets around the 'a are just a bit of syntactic cruft). Note that 'a is the type of foo#doSomething, not of foo.
# let x = new myClass (new myClass2);;
val x : unit myClass = <obj>
# x#doIt ;;
Doing something
- : unit = ()
Declare the type:
class myClass (foo:myClass2) =
Related
I wonder why one example fails and not the other.
(* this fails *)
(* (l fails to type check)
This expression has type 'a but an expression was expected of type
(module M.TFixU)
The module type M.TFixU would escape its scope
*)
let foldList1 (type ar) algr l =
let module M = FixT (ListIntF) in
let (module LU : M.TFixU) = l in
assert false
(* but this works *)
let foldList2 (type ar) algr l =
let (module LU : FixT(ListIntF).TFixU) = l in
assert false
complete code
module Higher = struct
type ('a, 't) app
module type NewType1 = sig
type 'a s
type t
val inj : 'a s -> ('a, t) app
val prj : ('a, t) app -> 'a s
end
module NewType1 (X : sig
type 'a t
end) =
struct
type 'a s = 'a X.t
type t
external inj : 'a s -> ('a, t) app = "%identity"
external prj : ('a, t) app -> 'a s = "%identity"
end
end
module Fix = struct
open Higher
module FixT (T : NewType1) = struct
module type T_Alg = sig
type a
val alg : (a, T.t) app -> a
end
module type TFixU = sig
module App : functor (A : T_Alg) -> sig
val res : A.a
end
end
type tFixU = (module TFixU)
end
end
module Pb = struct
open Higher
open Fix
(* intro *)
type 'r listIntF = Empty | Succ of (int * 'r)
module ListIntF = NewType1 (struct
type 'r t = 'r listIntF
end)
(* this fails *)
let foldList1 (type ar) algr l =
let module M = FixT (ListIntF) in
let (module LU : M.TFixU) = l in
(* (l fails to type check)
This expression has type 'a but an expression was expected of type
(module M.TFixU)
The module type M.TFixU would escape its scope
*)
let module T = LU.App (struct
type a = ar
let alg = algr
end) in
T.res
(* but this doesn't *)
let foldList2 (type ar) algr l =
let (module LU : FixT(ListIntF).TFixU) = l in
let module T = LU.App (struct
type a = ar
let alg = algr
end) in
T.res
end
In the first case, the type of l is unified with the type defined in the module M, which defines the module type. Since the type is introduced after the value l, which is a parameter in an eager language so it already exists, the value l receives a type that doesn't yet exist at the time of its creation. It is the soundness requirement of the OCaml type system that the value lifetime has to be enclosed with its type lifetime, or more simply each value must have a type. The simplest example is,
let x = ref None (* here `x` doesn't have a type since it is defined later *)
type foo = Foo;; (* the `foo` scope starts here *)
x := Some Foo (* foo escapes the scope as it is assigned to `x` via `foo option` *)
Another simplified example, that involves a function parameter is the following,
let foo x =
let open struct
type foo = Foo
end in
match x with
| Some Foo -> true (* again, type foo escapes the scope as it binds to `x` *)
| None -> false
A very good article that will help you understand in-depth scopes and generalization is Oleg Kiselyov's How OCaml type checker works -- or what polymorphism and garbage collection have in common.
Concerning the second case, you clearly specified the type of l using the applicative nature of OCaml functors. And since the typechecker knows that the lifetime of FixT(ListIntF).TFixU is greater than the lifetime of l it is happy.
I have very simple signature and module in OCaml:
module type S = sig
type t
val y : t
end;;
and
module M2 : S = struct
type t = int
let x = 1
let y = x+2
end;;
I cannot use construction like
M2.y
to get 3 unless i specify the module as
module M2 : S with type t = int = struct ...
Why is it so? There already is statement, that type t = int
The concrete, int value for M2.y is indeed not available because the following two conditions are met:
the type of y is abstract in the signature S
(there is no type t = ... there)
the module M2 is made opaque with respect to the signature S
(in other words, it is restricted to the signature S via the notation : S)
As a result, you indeed obtain:
let test = M2.y ;;
(* val test : M2.t = <abstr> *)
As suggested by the keyword <abstr>, this is related to the notion of abstract type. This notion is a very strong feature enforced by OCaml's typing rules, which prevents any user of a module having signature S to inspect the concrete content of one such abstract type. As a result, this property is very useful to implement so-called abstract data types (ADT) in OCaml, by carefully separating the implementation and the signature of the ADT.
If any of the two conditions above is missing, the type won't be abstract anymore and the concrete value of y will show up.
More precisely:
If the type t is made concrete, you obtain:
module type S = sig
type t = int
val y : t
end
module M2 : S = struct
type t = int
let x = 1
let y = x+2
end
let test = M2.y ;;
(* val test : M2.t = 3 *)
But in practice this is not very interesting because you lose generality. However, a somewhat more interesting approach consists in adding an "evaluator" or a "pretty-printer" function to the signature, such as the value int_of_t below:
module type S = sig
type t
val y : t
val int_of_t : t -> int
end
module M2 : S = struct
type t = int
let x = 1
let y = x+2
let int_of_t x = x
end
let test = M2.(int_of_t y) ;;
(* val test : int = 3 *)
Otherwise, if the module M2 is made transparent, you obtain:
module type S = sig
type t
val y : t
end
module M2 (* :S *) = struct
type t = int
let x = 1
let y = x+2
end
let test = M2.y ;;
(* val test : int = 3 *)
Finally, it may be helpful to note that beyond that feature of abstract types, OCaml also provides a feature of private types that can be viewed as a trade-off between concrete and abstract types used in a modular development. For more details on this notion, see for example Chap. 8 of Caml ref man.
I have defined two module types and two modules
module type FOO = sig type e end
module type BAR = sig type t end
module Foo : FOO = struct type e = int end
module Bar : BAR = struct type t = int end
Then I define a functor as
module Fun (F:FOO) (B:BAR with type t = F.e) = struct type x = string end
(this is a toy example, please ignore the fact that F and B are not used by the functor)
Now, if I define the module
module Bla = Fun (Foo) (Bar)
I get
Error: Signature mismatch:
Modules do not match:
sig type t = Bar.t end
is not included in
sig type t = Foo.e end
Type declarations do not match:
type t = Bar.t
is not included in
type t = Foo.e
Although both Bar.t and Foo.e are defined as int OCaml considers Bar.t and Foo.e to be different. That's just the way the typing system works and it makes sense to consider these two types different in general (c.f. last paragraph of Functors and Type Abstraction).
Question: Sometimes I may want this to pass type checking because for my purposes they can be considered equal. Is there a way to relax this?
Using gasche's suggestion of removing coercion, the above code can be written as
module type FOO = sig type e end
module type BAR = sig type t end
module Foo = struct type e = int end
module Bar = struct type t = int end
module Fun (F : FOO with type e=int) (B : BAR with type t = int) = struct type x = F.e * B.t end
module Bla = Fun (Foo) (Bar)
which compiles fine. Strangely, I get
# let f x : Bla.x = (x,x);;
val f : Foo.e -> Bla.x = <fun>
Question: why does it infer that x is Foo.e? It could as well be Bar.t?
The problem is how you define Foo and Bar : module Foo : FOO = .... By imposing this signature here, you "seal" the module and make the type abstract. It cannot be reverted. You should remove the : FOO coercion here, and use it later when you need the abstraction. You could also use module Foo : (FOO with type e = int) = ....
I'm not sure how the printer chooses amongst equal types, but in this case you can cause it to print a different name by explicitly annotating your function argument:
# let f (x:Bar.t) : Bla.x = (x,x);;
val f : Bar.t -> Bla.x = <fun>
I currently have two "layers" of modules that represent identifier-data relationships in a database.
The first layer defines identifier types, such as IdUser.t or IdPost.t while the second layer defines data types such as User.t or Post.t. I need all the modules of the first layer to be compiled before the modules of the second layer, because a Post.t must hold the IdUser.t of its author and the User.t holds the IdPost.t of the last five posts he visited.
Right now, IdUser.t provides functionality that should only ever be used by User.t, such as the ability to transform an IdUser.t into an IdUser.current: for security reasons, this transform must only ever be performed by the function User.check_password. Since IdUser and User are independent modules, I need to define those features as public functions and rely on conventions to avoid calling them anywhere outside of User, which is rather dirty. A symmetrical situation happens in IdPost.mine:
module IdUser : sig
type t
type current
val current_of_t : t -> current (* <--- Should not be public! *)
end = struct
type t = string
type current = string
let current_of_t x = x
end
module IdPost : sig
type t
type mine
val mine_of_t : t -> mine (* <--- Should not be public! *)
end = struct
type t = string
type mine = string
let mine_of_t x = x
end
module Post : sig
(* Should not "see" IdUser.current_of_t but needs IdPost.mine_of_t *)
val is_mine : IdUser.current -> IdPost.t -> IdPost.mine
end
module User : sig
(* Should not "see" IdPost.mine_of_t but needs IdUser.current_of_t *)
val check_password : IdUser.t -> password:string -> IdUser.current
end
Is there a way of defining an current_of_t : t -> current function in IdUser that can only be called from within module User ?
EDIT: this was a simplified example of one pair of modules, but there's an obvious solution for a single pair that cannot be generalized to multiple pairs and I need to solve this for multiple pairs — about 18 pairs, actually... So, I've extended it to be an example of two pairs.
So IdUser is in reality an existential type: For User there exists a type
IdUser.current such that the public IdUser.t can be lifted to it. There are a couple of ways to encode this: either functorize User as Gasche shows if statically managing the dependence is sufficient, or use first-class modules or objects if you need more dynamism.
I'll work out Gasche's example a bit more, using private type abbreviations for convenience and to show how to leverage translucency to avoid privatizing implementation types too much. First, and this might be a limitation, I want to declare an ADT of persistent IDs:
(* File id.ml *)
module type ID = sig
type t
type current = private t
end
module type PERSISTENT_ID = sig
include ID
val persist : t -> current
end
With this I can define the type of Posts using concrete types for the IDs but with ADTs to enforce the business rules relating to persistence:
(* File post.ml *)
module Post
(UID : ID with type t = string)
(PID : PERSISTENT_ID with type t = int)
: sig
val is_mine : UID.current -> PID.t -> PID.current
end = struct
let is_mine uid pid =
if (uid : UID.current :> UID.t) = "me" && pid = 0
then PID.persist pid
else failwith "is_mine"
end
The same thing with Users:
(* File user.ml *)
module User
(UID : PERSISTENT_ID with type t = string)
: sig
val check_password : UID.t -> password:string -> UID.current
end = struct
let check_password uid ~password =
if uid = "scott" && password = "tiger"
then UID.persist uid
else failwith "check_password"
end
Note that in both cases I make use of the concrete but private ID types. Tying all together is a simple matter of actually defining the ID ADTs with their persistence rules:
module IdUser = struct
type t = string
type current = string
let persist x = x
end
module IdPost = struct
type t = int
type current = int
let persist x = x
end
module MyUser = User (IdUser)
module MyPost = Post (IdUser) (IdPost)
At this point and to fully decouple the dependencies you will probably need signatures for USER and POST that can be exported from this module, but it's a simple matter of adding them in.
One way that seems to work at least on your simplified example is to group IdUser and User inside a same module:
module UserAndFriends : sig ... end = struct
module IdUser : sig
...
end = struct
...
end
module User = struct
...
end
end
module Post : sig
val create : (* <--- Should not "see" IdUser.current_of_t *)
author:IdUser.current -> title:string -> body:string -> IdPost.t
end
Hiding the dangerous function(s) in the signature of UserAndFriends gives the result you desire. If you do not want to make a big file containing both IdUser and User, you can use option -pack of ocamlc to create UserAndFriends. Note that in this case, you must craft your Makefile carefully so that the .cmi files of IdUser and User are not visible when compiling Post. I am not the Makefile specialist for Frama-C, but I think we use separate directories and position the compiler option -I carefully.
I suggest you parametrize Post (and possibly User for consistency) by a signature for the IdUser module : you would use a signature with current_of_t for User, and one without for Post.
This guarantee that Post doesn't use IdUser private features, but the public interface of IdUser is still too permissive. But with this setup, you have reversed the dependencies, and IdUser (the sensitive part) can control its use directly, give itself (with the private part) to IdUser and restrict the public signature to the public parts.
module type PrivateIdUser = sig
val secret : unit
end
module type PublicIdUser = sig
end
module type UserSig = sig
(* ... *)
end
module MakeUser (IdUser : PrivateIdUser) : UserSig = struct
(* ... *)
end
module IdUser : sig
include PublicIdUser
module User : UserSig
end
= struct
module IdUser = struct
let secret = ()
end
module User = MakeUser(IdUser)
include IdUser
end
module Post = struct
(* ... *)
end
Edit : Pascal Cuoq's concurrent -- in the temporal sense -- solution is alos very nice. Actually it's simpler and has less boilerplate. My solution adds an abstraction that allows for slightly more modularity, as you can define User independently of IdUser.
I think which solution is best probably depends on the specific application. If you have a lot of different modules that use PrivateIdUser private information, then using functors to write them separately instead of bundling everyone in the same module can be a good idea. If only User needs to be in the "private zone" and it's not very big, then Pascal's solution is a better choice.
Finally, while being forced to explicit Private and Public interfaces can be seen as an additional burden, it is also a way to make the access properties of different modules more explicit that using the position inside the module hierarchy.
It's possible to achieve fine-grained control over signatures with a combination of recursive modules, first-class modules and GADTs, but the limitation would be that all modules should then be inside the same top-level module and unpackings of first-class modules inside the recursive modules should be done in each function separately (not on the module-level as it would cause runtime exception Undefined_recursive_module):
module rec M1 : sig
module type M2's_sig = sig
val a : int
val c : float
end
module type M3's_sig = sig
val b : string
val c : float
end
type _ accessor =
| I'm_M2 : M2.wit -> (module M2's_sig) accessor
| I'm_M3 : M3.wit -> (module M3's_sig) accessor
val access : 'a accessor -> 'a
type wit
val do_it : unit -> unit
end = struct
module type M2's_sig = sig
val a : int
val c : float
end
module type M3's_sig = sig
val b : string
val c : float
end
type _ accessor =
| I'm_M2 : M2.wit -> (module M2's_sig) accessor
| I'm_M3 : M3.wit -> (module M3's_sig) accessor
module M1 = struct
let a = 1
let b = "1"
let c = 1.
end
let access : type a. a accessor -> a =
function
| I'm_M2 _ -> (module M1)
| I'm_M3 _ -> (module M1)
type wit = W
let do_it () =
let (module M2) = M2.(access ## I'm_M1 W) in
let (module M3) = M3.(access ## I'm_M1 W) in
Printf.printf "M1: M2: %d %s M3: %d %s\n" M2.a M2.b M3.a M3.b
end
and M2 : sig
module type M1's_sig = sig
val a : int
val b : string
end
module type M3's_sig = sig
val b : string
val c : float
end
type _ accessor =
| I'm_M1 : M1.wit -> (module M1's_sig) accessor
| I'm_M3 : M3.wit -> (module M3's_sig) accessor
val access : 'a accessor -> 'a
type wit
val do_it : unit -> unit
end = struct
module type M1's_sig = sig
val a : int
val b : string
end
module type M3's_sig = sig
val b : string
val c : float
end
type _ accessor =
| I'm_M1 : M1.wit -> (module M1's_sig) accessor
| I'm_M3 : M3.wit -> (module M3's_sig) accessor
module M2 = struct
let a = 2
let b = "2"
let c = 2.
end
let access : type a. a accessor -> a =
function
| I'm_M1 _ -> (module M2)
| I'm_M3 _ -> (module M2)
type wit = W
let do_it () =
let (module M1) = M1.(access ## I'm_M2 W) in
let (module M3) = M3.(access ## I'm_M2 W) in
Printf.printf "M2: M1: %d %f M3: %d %f\n" M1.a M1.c M3.a M3.c
end
and M3 : sig
module type M1's_sig = sig
val a : int
val b : string
end
module type M2's_sig = sig
val a : int
val c : float
end
type _ accessor =
| I'm_M1 : M1.wit -> (module M1's_sig) accessor
| I'm_M2 : M2.wit -> (module M2's_sig) accessor
val access : 'a accessor -> 'a
type wit
val do_it : unit -> unit
end = struct
module type M1's_sig = sig
val a : int
val b : string
end
module type M2's_sig = sig
val a : int
val c : float
end
type _ accessor =
| I'm_M1 : M1.wit -> (module M1's_sig) accessor
| I'm_M2 : M2.wit -> (module M2's_sig) accessor
module M3 = struct
let a = 3
let b = "3"
let c = 3.
end
let access : type a. a accessor -> a =
function
| I'm_M1 _ -> (module M3)
| I'm_M2 _ -> (module M3)
type wit = W
let do_it () =
let (module M1) = M1.(access ## I'm_M3 W) in
let (module M2) = M2.(access ## I'm_M3 W) in
Printf.printf "M3: M1: %s %f M2: %s %f\n" M1.b M1.c M2.b M2.c
end
let () =
M1.do_it ();
M2.do_it ();
M3.do_it ()
I'm having trouble with lists in OCaml. I've read conflicting statements saying whether or not the lists can be modified at runtime. Can the cons operator be used at runtime?
Additionally, why is a doberman (see below) allowed to be in a list of chihuahuas? How would one go about adding another chihuahua onto the list (as attempted with the last line)?
class virtual dog =
object
method virtual bark : unit
end;;
class chihuahua =
object
inherit dog
method bark = Printf.printf "Yip!"
end;;
class doberman =
object
inherit dog
method bark = Printf.printf "Roar!"
end;;
let c1 = new chihuahua;;
let c2 = new chihuahua;;
let c3 = new chihuahua;;
let d1 = new doberman;;
let arrayOfDogs = [c1;c2;d1];;
arrayOfDogs :: c3;;
1) You can use the cons operator at runtime, it just returns a new list rather than mutating the input list.
2) Class types in OCaml use "structural" subtyping, rather than Java-style "nominal" subtyping. The inferred type of arrayOfDogs will be "an object with a method named bark of type unit -> unit (not necessarily a dog)". For example:
# class cat = object
method bark = print_endline "meow"
end ;;
class cat : object method bark : unit end
# let c = new cat ;;
val c : cat = <obj>
# c :: arrayOfDogs ;;
- : cat list = [<obj>; <obj>; <obj>; <obj>]
3) The problem with arrayOfDogs :: c3 is you've got it the wrong way around. The type of :: is 'a -> 'a list -> 'a list. To add c3 at the beginning, use
c3 :: arrayOfDogs
To add it at the end, use the "append" operator #
arrayOfDogs # [c3]
you need to have your list on the right hand side, not the left. Ie:
c3 :: arrayOfDogs;;
This is why the last line fails.
As far as the list construction goes, given that OCaml is type-inferred, the interpreter probably figured out that you're constructing a list of dogs given you added the doberman on construction. Therefore it's not a list of Chihuahuas.
What does OCaml report as the type of arrayOfDogs?
Perhaps you mean: c3 :: arrayOfDogs;;