Let two variant types :
type typeA =
| A1
| A2
;;
type typeB =
| B1 of typeA
| B2 of typeA
;;
and type-checking functions :
let isA1 = function A1 -> true | _ -> false;;
let isA2 = function A2 -> true | _ -> false;;
let isB1 = function B1 e -> true | _ -> false;;
let isB2 = function B2 e -> true | _ -> false;;
I'd like to create a list of those functions to check elements of type A or B
as they're of different types, I need polymorphic variants and I get :
type filterA =
{
handleA : typeA -> bool;
};;
type filterB =
{
handleB : typeB -> bool;
};;
type filterslist = [`FilterA of filterA | `FilterB of filterB] list ;;
let filters1 = [`FilterA { handleA = isA1 }; `FilterB { handleB = isB1 }] ;;
So now I want to iterate over filters1 to check the type of the argument
I tried :
let exec_filters filters event = List.iter (fun fil -> match fil with `FilterA -> fil.handleA event; ()| `FilterB -> fil.handleB event; () ) filters;;
but it's not appreciated :
Error: This expression has type [< `FilterA | `FilterB ]
but an expression was expected of type filterA
How can I handle this ?
The fact that you're using "type checking predicates" similar to Scheme or instanceOf indicates that there is probably something very wrong with your code. OCaml is a statically typed language, you should not:
iterate over filters1 to check the type of the argument I tried
Why are you doing this? If you are trying to handle multiple types, the way to do it is to use polymorphism. Polymorphic variants can be helpful for this, but I'm still not convinced that your code isn't just written in a strange way.
I think your code should read like:
let exec_filters filters event =
List.iter
(fun fil -> match fil with
| `FilterA fA -> fA.handleA event; ()
| `FilterB fB -> fB.handleB event; () )
filters;;
EDIT: However, this won't typecheck, since event can't have types typeA and typeB...
Why not make your initial variants (typeA and typeB) polymorphic?
What are you trying to do?
When you say
match fil with
`FilterA -> ...
You seem to expect that this will change the type of fil, but that's not how it works. The expression with the type filterA appears inside the pattern. You want something more like this:
match fil with
`FilterA { handleA = h } -> h event
I'm not sure I see the purpose of having your handlers return bool if you're going to use List.iter to execute them. This will return unit, and the bool values are going to be discarded.
Edit
There's a deeper typing problem, explained well by Ptival. So even if you fix your patterns you'll still need to rethink your plan. One possible thing to do would be to use variants (not necessarily polymorphic variants, by the way) to track the types of the events.
Related
I'm trying to use the module type Partial_information which is constructed via the functor Make_partial_information as the type of the field contents in the type Cell.t. However, I'm getting the error Unbound module Partial_information.
open Core
(* module which is used as argument to functor *)
module type Partial_type = sig
type t
val merge : old:t -> new_:t -> t
end
(* type of result from functor *)
module type Partial_information = sig
type a
type t = a option
val merge : old:t -> new_:t -> t
val is_nothing : t -> bool
end
(* The functor *)
module Make_partial_information(Wrapping : Partial_type):
(Partial_information with type a = Wrapping.t)
= struct
type a = Wrapping.t
type t = a option
let merge ~(old : t) ~(new_ : t) =
match (old, new_) with
| (None, None) -> None
| (None, Some a) -> Some a
| (Some a, None) -> Some a
| (Some a, Some b) -> (Wrapping.merge ~old:a ~new_:b) |> Some
let is_nothing (it: t) : bool = (is_none it)
end
(* Checking to make sure understanding of functor is correct *)
module Int_partial_type = struct
type t = int
let merge ~old ~new_ = new_ [##warning "-27"]
end
module Int_partial_information = Make_partial_information(Int_partial_type)
(* Trying to use _any_ type which might have been created by the functor as a field in the record *)
module Cell = struct
type id = { name : string ; modifier : int }
type t = {
(* Error is here stating `Unbound module Partial_information` *)
contents : Partial_information.t ;
id : id
}
end
Module types are specifications for modules. They do not define types by themselves. They are also not constructed by functors in any way.
Consequently, it is hard to tell what you are trying to do.
As far I can see, you can simply define your cell type with a functor:
module Cell(P : Partial_information) = struct
type id = { name : string ; modifier : int }
type partial
type t = {
contents : P.t;
id : id
}
end
Or it might be even simpler to make the cell type polymorphic:
type 'a cell = {
contents : 'a;
id : id
}
since the type in itself is not particularly interesting nor really dependent upon
the type of contents.
P.S:
It is possible to use first class modules and GADTs to existentially quantify over a specific implementation of a module type. But it is unclear if it is worthwhile to explode your complexity budget here:
type 'a partial_information = (module Partial_information with type a = 'a)
module Cell = struct
type id = { name : string ; modifier : int }
type t = E: {
contents : 'a ;
partial_information_implementation: 'a partial_information;
id : id
} -> t
end
Say I have the following type and function:
exception NotFound
type 'a mytype = (string * 'a) list
fun foo [] = raise NotFound
| foo (x, b) :: bs = b
If I were to call the function and pass in an empty list, it will raise an Exception. Normally the function would return b, which is of type 'a. I want to handle the exception but I don't know what type 'a is, so what do I do here?
Related, but I really just want to return a bool. How do I do so?
Obviously the following code does not work as desired but it illustrates what I am trying to accomplish.
fun test mytypeobj = foo mytypeobj handle NotFound => false
(* return true otherwise *)
Your code contains a syntax error because of a left-parenthesis that comes a little too soon:
exception NotFound
type 'a mytype = (string * 'a) list
fun foo [] = raise NotFound
| foo (x, b :: bs) = b
I want to handle the exception but I don't know what type 'a is, so what do I do here?
The exception is unrelated to 'a, so handling it is a matter of:
val bar = foo [] handle NotFound => ...
I really just want to return a bool. How do I do so?
fun test mytypeobj = foo mytypeobj handle NotFound => false (* otherwise true *)
Since the function foo demonstrates no practical purpose, I am unsure what boolean you want to return. If you don't care if the boolean you're returning is the result of computing foo ..., you can do the following to discard the result and return a boolean of your choice:
fun test bs = (foo bs; true) handle NotFound => false
If b::bs is a list of booleans and you want to return the first of those, then that's what your function does. You can't be sure, of course, that it's true. The pattern I just mentioned is useful for unit testing when a function is throwing the right message. You could for example test that hd on the empty list correctly throws Empty:
fun test_hd_empty = (hd []; false) handle Empty => true
| _ => false
This test says that if hd [] doesn't throw, its result is discarded and the test fails. And if it does throw, but the exception isn't Empty, it should also fail.
I'm writing a tool that generates ml stubs from mli files. I have the meaningful mappings complete (var f : int -> float to let f _ = 0.), but I'm having a little trouble reasoning about the AST nodes for classes and modules, specifically the Pmty_* and Pcty_* nodes. These two types of nodes (given the type hierarchy) seem to most most associated with mli files. Some of the trivial members--which appear in -dparsetrees of mli files (like Pmty_ident and Pcty_constr) have obvious mappings to nodes most associated with ml files (again by the hierarchy of the signature and structure types, in the aforementioned example to Pmod_ident and Pcl_constr). However, some of the nodes don't have an obvious parallel. Specifically, I'm having trouble reasoning about:
Pmty_with and Pmty_typeof - it seems like these two can never be parsed from a valid mli file; they only occur in a structure context that has a module_type as one of its members (I've checked my suspicions against all of OCaml's parser test files)
Pcty_signature - it seems like the only case this can occur is within a Psig_class_type (which I already directly map to Pstr_class_type); I against the OCaml test files and found a Pcty_signature only in this location, but are there other valid locations for it in an mli that I am missing?
Pcty_arrow - there are no test files containing this, so I can't be sure where it is valid, but my intuition says this also goes in a class_type context within a structure only (or within one of the other Pcty_*, in which case its conversion would be handled as a child by that node's mapping from Pcty_* to Pcl_*); is this incorrect?
I'm fairly deep into this and don't completely understand what's going on with all of these advanced language features and the AST nodes representing them, so here's an attempt at a simpler explanation of my questions:
Relevant types extracted from Parsetree:
type module_type_desc =
(* .. snip .. *)
| Pmty_with of module_type * with_constraint list
(* MT with ... *)
| Pmty_typeof of module_expr
(* module type of ME *)
type class_type_desc =
(* .. snip .. *)
| Pcty_signature of class_signature
(* object ... end *)
| Pcty_arrow of arg_label * core_type * class_type
(* T -> CT Simple
~l:T -> CT Labelled l
?l:T -> CT Optional l
*)
I believe that Pmty_with and Pmty_typeof can only occur in ml files (and not mli files). Is this assumption correct?
Can Pcty_signature occur as a node that isn't the child of a Psig_class_type?
Can a Pcty_arrow occur in a valid mli file? Where? As a child of what?
As I mentioned before, I'm fairly confident of my handling of every besides these two (modules and classes). In case the above isn't clear, here's an annotated snippet of the code that transforms Parsetree.signature -> Parsetree.structure with all of the non-module/class stuff removed for brevity:
(* Parsetree.signature -> Parsetree.structure *)
let rec stub signature_items =
(* Handles the module_type_desc *)
let rec stub_module_type module_type =
match module_type with
| { pmty_desc = type_; pmty_attributes = attrs; _ } ->
let expr =
match type_ with
| Pmty_ident ident -> Pmod_ident ident
| Pmty_signature signatures -> Pmod_structure (stub signatures)
| Pmty_functor (name, a, b) -> Pmod_functor (name, a, (stub_module_type b))
(* XXX: unclear if these two can occur in an mli *)
(* | Pmty_with (type_, constraints) -> _ TODO *)
(* | Pmty_typeof type_ -> _ TODO *)
| Pmty_extension ext -> Pmod_extension ext
| Pmty_alias name -> Pmod_ident name
in
make_module_expr expr attrs
in
(* The next three functions handles the module_type for single and multiple (rec) modules *)
let stub_module_decl module_decl =
match module_decl with
| { pmd_name = name; pmd_type = type_; pmd_attributes = attrs; _ } ->
make_module_binding name (stub_module_type type_) attrs
in
let stub_module module_ = Pstr_module (stub_module_decl module_)
and stub_modules modules = Pstr_recmodule (List.map stub_module_decl modules)
and stub_include include_ =
match include_ with
| { pincl_mod = module_type; pincl_attributes = attrs; _ } ->
Pstr_include (make_include_decl (stub_module_type module_type) attrs)
in
(* Handles classes (class_type) *)
let stub_classes classes =
(* Handles class_type_desc *)
let stub_class_descr descr =
let rec stub_class class_ =
let stub_class_type type_ =
match type_ with
| Pcty_constr (ident, types) -> Pcl_constr (ident, types)
| Pcty_signature class_ -> (* XXX: Is my below assumption true? *)
failwith "should be covered by Psig_class_type -> Pstr_class_type"
(* XXX: do we ever need to handle Pcty_arrow for mli files? *)
(* | Pcty_arrow (label, a, b) -> _ *)
| Pcty_extension ext -> Pcl_extension ext
| Pcty_open (override, ident, class_) ->
Pcl_open (override, ident, (stub_class class_))
in
match class_ with
| { pcty_desc = type_; pcty_attributes = attrs; _ } ->
make_class_expr (stub_class_type type_) attrs
in
match descr with
| { pci_virt = virt; pci_params = params; pci_name = name;
pci_expr = class_; pci_attributes = attrs } ->
make_class_decl virt params name (stub_class class_) attrs
in
Pstr_class (List.map stub_class_descr classes)
in
let transform_signature signature_item =
match signature_item with
| { psig_desc = signature; _ } ->
let desc =
match signature with
(* ... clip non-module/class stuff ... *)
| Psig_module module_ -> stub_module module_
| Psig_recmodule modules -> stub_modules modules
| Psig_include include_ -> stub_include include_
| Psig_class classes -> stub_classes classes
| Psig_class_type classes -> Pstr_class_type classes
in
make_str desc
in
List.map transform_signature signature_items
Unfortunately the module/class stuff is rather complex logic, so trimmed down there's still a lot. There are a ton of helps for creating the *_desc wrappers that encapsulate location in the file, attributes, etc., but those shouldn't be key to understanding how I'm handling modules and classes. But just for clarity, here are the types of all of the helpers:
val make_str : Parsetree.structure_item_desc -> Parsetree.structure_item
val make_module_expr :
Parsetree.module_expr_desc -> Parsetree.attributes -> Parsetree.module_expr
val make_module_binding :
string Asttypes.loc ->
Parsetree.module_expr -> Parsetree.attributes -> Parsetree.module_binding
val make_include_decl :
'a -> Parsetree.attributes -> 'a Parsetree.include_infos
val make_class_decl :
Asttypes.virtual_flag ->
(Parsetree.core_type * Asttypes.variance) list ->
string Asttypes.loc ->
'a -> Parsetree.attributes -> 'a Parsetree.class_infos
val make_class_expr :
Parsetree.class_expr_desc -> Parsetree.attributes -> Parsetree.class_expr
Relevant docs:
parsetree.ml (better than docs, because of some comments)
Edit: As an aside, besides reading documentation on these features (which didn't yield any AST patterns I didn't already know about), I recalled that the compiled can derive the interface from the implementation ocamlc -i. I traced down the variable in the compiler (it's called print_types) that's linked to this flag and found all of its uses, but it was not immediately apparent to me where at any of its uses code is called that derives the mli file (perhaps it is done progressively with the parse, since compiling produces a cmi?). If someone with more OCaml chops or more experience with the compiler could point me to where the mli file is derived, it may be easier to reverse engineer these module and class AST nodes.
Edit 2: I am also aware of How to auto-generate stubs from mli file?, however the answer there is "do it manually," which definitely conflicts with what I'm attempting! (The answerer also claims that such a tool would be trivial, but after pouring over these AST nodes for a while, I beg to differ!)
(I did not include the -dparsetree in my answer as it is heavy and not that interesting).
I believe that Pmty_with and Pmty_typeof can only occur in ml files (and not mli files). Is this assumption correct?
module M : module type of struct type t end with type t = int
As you can see from this valid mli file, this assumption isn't correct. .mli files require the same parser as .ml files do.
Can Pcty_signature occur as a node that isn't the child of a Psig_class_type?
class type c = object inherit object method x : int end end
Yes it can. Pcty_signature can occur anywhere a class type can occur. (note that there are two Pcty_signature here, one is the child of Pctf_inherit).
a Pcty_arrow occur in a valid mli file? Where? As a child of what?
class c' : int -> object method x : int end
Yes it can! And it can occur anywhere you'd indicate a class type.
Basically, you can consider that if a constructor can happen somewhere, then all the constructors of the same type can happen there too. Any type-related constructor can be in a .mli file (and non-type related constructors can happen too through the devious module type of).
If you have questions about where those are constructed, just take a look at parser.mly. Note that the same parser is used for the two file types.
This is a great game. Give me a list of AST Nodes, I'll write you a file that uses them all. :D
module K : module type of String
module M : Map.S with type key = K.t
class fakeref : K.t -> object
method get : K.t
method set : K.t -> unit
end
So, to summarize: Classes can take arguments, hence Pcty_arrow. Pcty_signature can also be the child of Psig_class, as shown above. The other two are standard module constructions that can absolutely appear in .mli files.
As for how ocamlc -i works ... well, it returns the signature inferred by the typechecker. There is no single point of access to this. You can read typing/HACKING.md if you want, but beware, the rabbit hole goes very deep. That being said, I do not think this will be all that helpful to achieve your goal.
My advice would be the following: all the nodes above are fairly easy to handle, except for with_type. This one is very hard, because it basically allows to compute in signatures. Just give up on that one for now.
Also, be aware that values, modules, module types, class and class types all have different namespaces. Pmty_ident x -> Pmod_ident x is not correct.
I had this idea of a type-safe templating language that would use polymorphic variants as a source of type-safe variables that can be substituted for text, for example:
type 'a t = Var of 'a | Text of string | Join of 'a t * 'a t
let rec render ~vars = function
| Text source -> source
| Var label -> vars label
| Join (left, right) -> render left ~vars ^ render right ~vars
let result = render (Join (Var `Foo, Text "bar")) ~vars:(function `Foo -> "foo");;
let () = assert (result = "foobar")
This is all fine: compiler will enforce that you don't forget a substitution variable, or that you don't have a typo in a variable name—thanks to polymorphic variants.
However, I find two problems:
You can accidentally supply an unused variable.
If template contains no variables, you are still forced to supply a ~vars function, and the only one that would work would be fun _ -> "" or fun _ -> assert false, which compromizes type-safety in case the template ever changes.
I'm looking for advice on the problems above, but I also appreciate any applicable advice on API design.
Nothing force you to always use polymorphic variants. you could have a void type that is guaranteed to be different to every polymorphic variant.
type void
let empty_vars : void -> string = fun _ assert false
When you apply it to an empty template, you end up with
let result = render (Text "bar") ~vars:empty_vars
That way, if you later add a variable to your template, you will immediately notice it through the type error.
For unused variables, the best I can suggest is also not to use polymorphic variants:
type v = Foo
let result = render (Join (Var Foo, Text "bar")) ~vars:(function Foo -> "foo");;
This will only catch unused cases in the function definition, but of course if you remove a part of your template, you won't notice anything.
One other solution that have similar properties but may, or may not suit your taste is to use objects.
let rec render ~vars = function
| Text source -> source
| Var label -> label vars
| Join (left, right) -> render left ~vars ^ render right ~vars
let foo v = v#foo
let result = render (Join (Var foo, Text "bar")) ~vars:object method foo = "foo" end
That way you can keep the same pattern when no variables are used:
let result = render (Text "bar") ~vars:object end
But still no unused variable check.
I think it is impossible with polymorphic variants. The type of render function is:
val render : var:('a -> string) -> 'a t -> string
and the partial application render (Join (Var `Foo, Text "var")) has the following type:
vars:([> `Foo ] -> string) -> string
What you want to do is to close the opened variant type [> `Foo ] and restrict it to [ `Foo ] -> string in order to exclude functions which can get larger inputs like [< `Foo | `Bar ] -> string.
The only way to restrict the type is to add a type constraint: (vars : [ `Foo ] -> string), listing all the tags you want explicitly, but this is what you want to avoid...
In OCaml, it is possible to define explicit polymorphic type in a record
type foo = { f : 'a. unit -> 'a };;
It seems we can assign only general values to f like
{ f = fun () -> failwith ""; }
or
{ f = fun () -> exit 1; }
How to use this language feature in real world? Is there any good practical example?
This isn't really connected with records. If you declare any function to have type 'a. unit -> 'a (takes nothing and returns whatever the caller wanted) then you can only use it for functions that don't return.
Here's a slightly more useful example: a record containing a function for finding the length of lists (of any type).
# type foo = { f : 'a. 'a list -> int };;
type foo = { f : 'a. 'a list -> int; }
# let foo = { f = List.length };;
val foo : foo = {f = <fun>}
# foo.f [1;2;3];;
- : int = 3
It can be useful if you wanted to pass a function like List.length as an argument to another function, and have it use it on multiple types:
Say we want to pass List.length to test. We can't do it directly:
# let test fn = fn [1;2;3] + fn ["a";"b";"c"];;
Error: This expression has type string but an expression was expected of type
int
But we can use a record:
# let test foo = foo.f [1;2;3] + foo.f ["a";"b";"c"];;
val test : foo -> int = <fun>
# test foo;;
- : int = 6