I have defined a big pretty printer pp: out_channel -> t -> unit over a big type t. Therefore, I can use it like Printf.fprintf stdout "%a" x where x: t, or chain printing like Printf.fprintf chan "%a" pp x where chan: out_channel.
Now I need to convert what is printed to a string or a text. Does anyone know if there is a way to leverage/use the function pp rather than writing a function to_string: t -> unit from scratch?
Format.asprintf should suit your needs, if you pp is implemented for Format.formatter type instead of out_channel. The Format.formatter is a more general type and should be preferred to the concrete out_channel. In fact, a sort of a standard type for pretty printer is the Format.formatter -> 'a -> unit type, at least it is required by the #install_printer directive in OCaml toplevel, debugger and other facilities. Functions of the same type are used in Core library to implement Pretty_printer interface.
So, if you will reimplement your pp function to work with the Format module (usually for this it would be enough just to open Format module), then you can reuse it. The functions, that print to out_channel module can't be retargetered to print into string. So it is better not to write them.
To make this work you need something that looks to OCaml like an out channel, but keeps the data in a string (or buffer) instead. There's nothing like this in OCaml, unfortunately.
Related
I am using a 3rd party module which exposes a function:
val pp : Format.formatter -> 'a -> unit
Unfortunately it doesn't expose a to_string (or show) function.
I want to find a way to use the result of pp in a format string, something like:
let output = Format.sprintf "Result: %s" (SomeModule.pp fmt myval)
But pp writes to fmt and returns unit so of course this is not valid.
I can tell I need to somehow make a formatter to pass to pp that writes to a string buffer, that I can then get contents of as a string, which I can then pass as an arg to sprintf
The use of pp like functions for making types printable seems pretty ubiquitous in OCaml (e.g. ppx_deriving show generates them) so I feel like there should be a simple way to achieve this, but I'm currently missing it.
By using asprintf instead, it's possible to use the %a format specifier to pass two arguments, a printer function and the value to be printed, which will then be formatted accordingly and inserted in its place:
let output = Format.asprintf "Result: %a" SomeModule.pp myval
The reason asprintf has to be used instead of sprintf is that the latter specifies an "input source" (the second argument of the format type) of type unit, while the former uses a formatter. This is what's going to be passed to the printer.
It still alludes me why there's a need for sprintf though, rather than just having asprintf. Perhaps there's some performance-related reason for it, but my guess is that it's just an artifact of history.
I was desperately looking for the last hour for a method in the OCaml Library which converts an 'a to a string:
'a -> string
Is there something in the library which I just haven't found? Or do I have to do it different (writing everything by my own)?
It is not possible to write a printing function show of type 'a -> string in OCaml.
Indeed, types are erased after compilation in OCaml. (They are in fact erased after the typechecking which is one of the early phase of the compilation pipeline).
Consequently, a function of type 'a -> _ can either:
ignore its argument:
let f _ = "<something>"
peek at the memory representation of a value
let f x = if Obj.is_block x then "<block>" else "<immediate>"
Even peeking at the memory representation of a value has limited utility since many different types will share the same memory representation.
If you want to print a type, you need to create a printer for this type. You can either do this by hand using the Fmt library (or the Format module in the standard library)
type tree = Leaf of int | Node of { left:tree; right: tree }
let pp ppf tree = match tree with
| Leaf d -> Fmt.fp ppf "Leaf %d" d
| Node n -> Fmt.fp ppf "Node { left:%a; right:%a}" pp n.left pp n.right
or by using a ppx (a small preprocessing extension for OCaml) like https://github.com/ocaml-ppx/ppx_deriving.
type tree = Leaf of int | Node of { left:tree; right: tree } [##deriving show]
If you just want a quick hacky solution, you can use dump from theBatteries library. It doesn't work for all cases, but it does work for primitives, lists, etc. It accesses the underlying raw memory representation, hence is able to overcome (to some extent) the difficulties mentioned in the other answers.
You can use it like this (after installing it via opam install batteries):
# #require "batteries";;
# Batteries.dump 1;;
- : string = "1"
# Batteries.dump 1.2;;
- : string = "1.2"
# Batteries.dump [1;2;3];;
- : string = "[1; 2; 3]"
If you want a more "proper" solution, use ppx_deriving as recommended by #octachron. It is much more reliable/maintainable/customizable.
What you are looking for is a meaningful function of type 'a. 'a -> string, with parametric polymorphism (i.e. a single function that can operate the same for all possible types 'a, even those that didn’t exist when the function was created). This is not possible in OCaml. Here are explications depending on your programming background.
Coming from Haskell
If you were expecting such a function because you are familiar with the Haskell function show, then notice that its type is actually show :: Show a => a -> String. It uses an instance of the typeclass Show a, which is implicitly inserted by the compiler at call sites. This is not parametric polymorphism, this is ad-hoc polymorphism (show is overloaded, if you want). There is no such feature in OCaml (yet? there are projects for the future of the language, look for “modular implicits” or “modular explicits”).
Coming from OOP
If you were expecting such a function because you are familiar with OO languages in which every value is an object with a method toString, then this is not the case of OCaml. OCaml does not use the object model pervasively, and run-time representation of OCaml values retains no (or very few) notion of type. I refer you to #octachron’s answer.
Again, toString in OOP is not parametric polymorphism but overloading: there is not a single method toString which is defined for all possible types. Instead there are multiple — possibly very different — implementations of a method of the same name. In some OO languages, programmers try to follow the discipline of implementing a method by that name for every class they define, but it is only a coding practice. One could very well create objects that do not have such a method.
[ Actually, the notions involved in both worlds are pretty similar: Haskell requires an instance of a typeclass Show a providing a function show; OOP requires an object of a class Stringifiable (for instance) providing a method toString. Or, of course, an instance/object of a descendent typeclass/class. ]
Another possibility is to use https://github.com/ocaml-ppx/ppx_deriving with will create the function of Path.To.My.Super.Type.t -> string you can then use with your value. However you still need to track the path of the type by hand but it is better than nothing.
Another project provide feature similar to Batterie https://github.com/reasonml/reason-native/blob/master/src/console/README.md (I haven't tested Batterie so can't give opinion) They have the same limitation: they introspect the runtime encoding so can't get something really useable. I think it was done with windows/browser in mind so if cross plat is required I will test this one before (unless batterie is already pulled). and even if the code source is in reason you can use with same API in OCaml.
in F#, the following is a no brainer:
let l = [1;2;3;4]
let s = sprintf "%A" l
where "%A" prints a formatted version of virtually any common, even recursive data structure.
Is there something similarly easy in ocaml?
There is something close, the %a specificator accepts two arguments, the first is a pretty printer for type 'a, and the second is a value of type 'a. The type of the printer, depends on the kind of used printf function. For example,
open Core_kernel.Std
open Format
printf "%a" Int63.pp Int63.one
Of course, this depends heavily on a good support from a library. If there is no pp function, provided for the type, then it is pretty useless.
Also there is a custom_printf syntax extension available for both - pp and ppx. In this extension you place a module name in the place of specificator. The module must have a to_string function. The ppx version, requires an exclamation mark before the format string:
printf !"%{Int63}" Int63.one
There is also a dump function, available over the Internet. In particular you can find it in the Batteries library. It recurse over the data representation and print it in a more or less human readable representation. But this is not relate to the formatted output.
Can someone give a concise description of when the relaxed value restriction kicks in? I've had trouble finding a concise and clear description of the rules. There's Garrigue's paper:
http://caml.inria.fr/pub/papers/garrigue-value_restriction-fiwflp04.pdf
but it's a little dense. Anyone know of a pithier source?
An Addendum
Some good explanations were added below, but I was unable to find an explanation there for the following behavior:
# let _x = 3 in (fun () -> ref None);;
- : unit -> 'a option ref = <fun>
# let _x = ref 3 in (fun () -> ref None);;
- : unit -> '_a option ref = <fun>
Can anyone clarify the above? Why does the stray definition of a ref within the RHS of the enclosing let affect the heuristic.
I am not a type theorist, but here is my interpretation of Garrigue's explanation. You have a value V. Start with the type that would be assigned to V (in OCaml) under the usual value restriction. There will be some number (maybe 0) monomorphic type variables in the type. For each such variable that appears only in covariant position in the type (on the right sides of function arrows), you can replace it with a fully polymorphic type variable.
The argument goes as follows. Since your monomorphic variable is a variable, you can imagine replacing it with any single type. So you choose an uninhabited type U. Now since it is in covariant position only, U can in turn be replaced by any supertype. But every type is a supertype of an uninhabited type, hence it's safe to replace with a fully polymorphic variable.
So, the relaxed value restriction kicks in when you have (what would be) monomorphic variables that appear only in covariant positions.
(I hope I have this right. Certainly #gasche would do better, as octref suggests.)
Jeffrey provided the intuitive explanation of why the relaxation is correct. As to when it is useful, I think we can first reproduce the answer octref helpfully linked to:
You may safely ignore those subtleties until, someday, you hit a problem with an abstract type of yours that is not as polymorphic as you would like, and then you should remember than a covariance annotation in the signature may help.
We discussed this on reddit/ocaml a few months ago:
Consider the following code example:
module type S = sig
type 'a collection
val empty : unit -> 'a collection
end
module C : S = struct
type 'a collection =
| Nil
| Cons of 'a * 'a collection
let empty () = Nil
end
let test = C.empty ()
The type you get for test is '_a C.collection, instead of the 'a C.collection that you would expect. It is not a polymorphic type ('_a is a monomorphic inference variable that is not yet fully determined), and you won't be happy with it in most cases.
This is because C.empty () is not a value, so its type is not generalized (~ made polymorphic). To benefit from the relaxed value restriction, you have to mark the abstract type 'a collection covariant:
module type S = sig
type +'a collection
val empty : unit -> 'a collection
end
Of course this only happens because the module C is sealed with the signature S : module C : S = .... If the module C was not given an explicit signature, the type-system would infer the most general variance (here covariance) and one wouldn't notice that.
Programming against an abstract interface is often useful (when defining a functor, or enforcing a phantom type discipline, or writing modular programs) so this sort of situation definitely happens and it is then useful to know about the relaxed value restriction.
That's an example of when you need to be aware of it to get more polymorphism, because you set up an abstraction boundary (a module signature with an abstract type) and it doesn't work automatically, you have explicitly to say that the abstract type is covariant.
In most cases it happens without your notice, when you manipulate polymorphic data structures. [] # [] only has the polymorphic type 'a list thanks to the relaxation.
A concrete but more advanced example is Oleg's Ber-MetaOCaml, which uses a type ('cl, 'ty) code to represent quoted expressions which are built piecewise. 'ty represents the type of the result of the quoted code, and 'cl is a kind of phantom region variable that guarantees that, when it remains polymorphic, the scoping of variable in quoted code is correct. As this relies on polymorphism in situations where quoted expressions are built by composing other quoted expressions (so are generally not values), it basically would not work at all without the relaxed value restriction (it's a side remark in his excellent yet technical document on type inference).
The question why the two examples given in the addendum are typed differently has puzzled me for a couple of days. Here is what I found by digging into the OCaml compiler's code (disclaimer: I'm neither an expert on OCaml nor on the ML type system).
Recap
# let _x = 3 in (fun () -> ref None);; (* (1) *)
- : unit -> 'a option ref = <fun>
is given a polymorphic type (think ∀ α. unit → α option ref) while
# let _x = ref 3 in (fun () -> ref None);; (* (2) *)
- : unit -> '_a option ref = <fun>
is given a monomorphic type (think unit → α option ref, that is, the type variable α is not universally quantified).
Intuition
For the purposes of type checking, the OCaml compiler sees no difference between example (2) and
# let r = ref None in (fun () -> r);; (* (3) *)
- : unit -> '_a option ref = <fun>
since it doesn't look into the body of the let to see if the bound variable is actually used (as one might expect). But (3) clearly must be given a monomorphic type, otherwise a polymorphically typed reference cell could escape, potentially leading to unsound behaviour like memory corruption.
Expansiveness
To understand why (1) and (2) are typed the way they are, let's have a look at how the OCaml compiler actually checks whether a let expression is a value (i.e. "nonexpansive") or not (see is_nonexpansive):
let rec is_nonexpansive exp =
match exp.exp_desc with
(* ... *)
| Texp_let(rec_flag, pat_exp_list, body) ->
List.for_all (fun vb -> is_nonexpansive vb.vb_expr) pat_exp_list &&
is_nonexpansive body
| (* ... *)
So a let-expression is a value if both its body and all the bound variables are values.
In both examples given in the addendum, the body is fun () -> ref None, which is a function and hence a value. The difference between the two pieces of code is that 3 is a value while ref 3 is not. Therefore OCaml considers the first let a value but not the second.
Typing
Again looking at the code of the OCaml compiler, we can see that whether an expression is considered expansive determines how the type of the let-expressions is generalised (see type_expression):
(* Typing of toplevel expressions *)
let type_expression env sexp =
(* ... *)
let exp = type_exp env sexp in
(* ... *)
if is_nonexpansive exp then generalize exp.exp_type
else generalize_expansive env exp.exp_type;
(* ... *)
Since let _x = 3 in (fun () -> ref None) is nonexpansive, it is typed using generalize which gives it a polymorphic type. let _x = ref 3 in (fun () -> ref None), on the other hand, is typed via generalize_expansive, giving it a monomorphic type.
That's as far as I got. If you want to dig even deeper, reading Oleg Kiselyov's Efficient and Insightful Generalization alongside generalize and generalize_expansive may be a good start.
Many thanks to Leo White from OCaml Labs Cambridge for encouraging me to start digging!
Although I'm not very familiar with this theory, I have asked a question about it.
gasche provided me with a concise explanation. The example is just a part of OCaml's map module. Check it out!
Maybe he will be able to provide you with a better answer. #gasche
Perhaps I'm going about this the wrong way, but I'm using HXT to read in some vertex data that I'd like to use in an array in HOpenGL. Vertex arrays need to be a Ptr which is created by calling newArray. Unfortunately newArray returns an IO Ptr, so I'm not sure how to go about using it inside an Arrow. I think I need something with a type declaration similar to IO a -> Arrow a?
The type IO a -> Arrow a doesn't make sense; Arrow is a type class, not a specific type, much like Monad or Num. Specifically, an instance of Arrow is a type constructor taking two parameters that describes things that can be composed like functions, matching types end-to-end. So, converting IO a to an arrow could perhaps be called a conceptual type error.
I'm not sure exactly what you're trying to do, but if you really want to be using IO operations as part of an Arrow, you need your Arrow instance to include that. The simplest form of that is to observe that functions with types like a -> m b for any Monad instance can be composed in the obvious way. The hxt package seems to provide a more complicated type:
newtype IOSLA s a b = IOSLA { runIOSLA :: s -> a -> IO (s, [b]) }
This is some mixture of the IO, State, and [] monads, attached to a function as above such that you can compose them going through all three Monads at each step. I haven't really used hxt much, but if these are the Arrows you're working with, it's pretty simple to lift an arbitrary IO function to serve as one--just pass the state value s through unchanged, and turn the output of the function into a singleton list. There may already be a function to do this for you, but I didn't see one at a brief glance.
Basically, you'd want something like this:
liftArrIO :: (a -> IO b) -> IOSLA s a b
liftArrIO f = IOSLA $ \s x -> fmap (\y -> (s, [y])) (f x)