Module BatPervasives

module BatPervasives: sig .. end
Additional functions.

Author(s): Xavier Leroy (Base module), Nicolas Cannasse, David Teller, Zheng Li


The initially opened module.

This module provides the basic operations over the built-in types (numbers, booleans, strings, exceptions, references, lists, arrays, input-output channels, ...)

This module is automatically opened at the beginning of each compilation. All components of this module can therefore be referred by their short name, without prefixing them by BatPervasives.

val input_lines : Pervasives.in_channel -> string BatEnum.t
Returns an enumeration over lines of an input channel, as read by the input_line function.
val input_chars : Pervasives.in_channel -> char BatEnum.t
Returns an enumeration over characters of an input channel.
val input_list : Pervasives.in_channel -> string list
Returns the list of lines read from an input channel.
val input_all : Pervasives.in_channel -> string
Return the whole contents of an input channel as a single string.
val dump : 'a -> string
Attempt to convert a value to a string.

Works well for a lot of cases such as non-empty lists, algebraic datatype, and records.

However, since types are lost at compile-time, the representation might not match your type. (0, 1) will be printed as expected, but (1, 0) and 1 have the same representation and will get printed in the same way. The result of dump is unspecified and may change in future versions, so you should only use it for debugging and never have program behavior depend on the output.

Here is a list of some of the surprising corner cases of the current implementation:

dump may fail for ill-formed values, such as obtained from a faulty C binding or crazy uses of Obj.set_tag.
val print_any : 'b BatIO.output -> 'a -> unit
Attempt to print a value to an output.

Uses dump to convert the value to a string and prints that string to the output.


List operations

More list operations are provided in module List.

val (@) : 'a list -> 'a list -> 'a list
List concatenation.

Input/output

This section only contains the most common input/output operations. More operations may be found in modules BatIO and File.

val stdin : BatIO.input
Standard input, as per Unix/Windows conventions (by default, keyboard).

Use this input to read what the user is writing on the keyboard.

val stdout : unit BatIO.output
Standard output, as per Unix/Windows conventions (by default, console).

Use this output to display regular messages.

val stderr : unit BatIO.output
Standard error output, as per Unix/Windows conventions.

Use this output to display warnings and error messages.

val stdnull : unit BatIO.output
An output which discards everything written to it.

Use this output to ignore messages.

val flush_all : unit -> unit
Write all pending data to output channels, ignore all errors.

It is normally not necessary to call this function, as all pending data is written when an output channel is closed or when the program itself terminates, either normally or because of an uncaught exception. However, this function is useful for debugging, as it forces pending data to be written immediately.


Output functions on standard output

val print_bool : bool -> unit
Print a boolean on standard output.
val print_guess : 'a BatIO.output -> 'b -> unit
Attempt to print the representation of a runtime value on the standard output. See remarks for BatPervasives.dump. This function is useful mostly for debugging. As a general rule, it should not be used in production code.
val print_all : BatIO.input -> unit
Print the contents of an input to the standard output.

Output functions on standard error

val prerr_bool : bool -> unit
Print a boolean to stderr.
val prerr_guess : 'a -> unit
Attempt to print the representation of a runtime value on the error output. See remarks for BatPervasives.dump. This function is useful mostly for debugging.
val prerr_all : BatIO.input -> unit
Print the contents of an input to the error output.

General output functions

val output_file : filename:string -> text:string -> unit
creates a filename, write text into it and close it.
val open_out : ?mode:BatFile.open_out_flag list ->
?perm:BatFile.permission -> string -> unit BatIO.output
Open the named file for writing, and return a new output channel on that file. You will need to close the file once you have finished using it.

You may use optional argument mode to decide whether the output will overwrite the contents of the file (by default) or to add things at the end of the file, whether the file should be created if it does not exist yet (the default) or not, whether this operation should proceed if the file exists already (the default) or not, whether the file should be opened as text (the default) or as binary, and whether the file should be opened for non-blocking operations.

You may use optional argument perm to specify the permissions of the file, as per Unix conventions. By default, files are created with default permissions (which depend on your setup).
Raises Sys_error if the file could not be opened.

val open_out_bin : string -> unit BatIO.output
Same as BatPervasives.open_out, but the file is opened in binary mode, so that no translation takes place during writes. On operating systems that do not distinguish between text mode and binary mode, this function behaves like BatPervasives.open_out without any mode or perm.
val open_out_gen : Pervasives.open_flag list -> int -> string -> unit BatIO.output
Deprecated.Use open_out instead
open_out_gen mode perm filename opens the named file for writing, as described above. The extra argument mode specifies the opening mode. The extra argument perm specifies the file permissions, in case the file must be created.
val flush : unit BatIO.output -> unit
Flush the buffer associated with the given output, performing all pending writes on that channel. Interactive programs must be careful about flushing standard output and standard error at the right time.
val output_char : unit BatIO.output -> char -> unit
Write the character on the given output channel.
val output_string : unit BatIO.output -> string -> unit
Write the string on the given output channel.
val output : unit BatIO.output -> string -> int -> int -> unit
output oc buf pos len writes len characters from string buf, starting at offset pos, to the given output channel oc.
Raises Invalid_argument if pos and len do not designate a valid substring of buf.
val output_byte : unit BatIO.output -> int -> unit
Write one 8-bit integer (as the single character with that code) on the given output channel. The given integer is taken modulo 256.
val output_binary_int : unit BatIO.output -> int -> unit
Write one integer in binary format (4 bytes, big-endian) on the given output channel. The given integer is taken modulo 232. The only reliable way to read it back is through the Pervasives.input_binary_int function. The format is compatible across all machines for a given version of OCaml.
val output_binary_float : unit BatIO.output -> float -> unit
Write one float in binary format (8 bytes, IEEE 754 double format) on the given output channel. The only reliable way to read it back is through the Pervasives.input_binary_float function. The format is compatible across all machines for a given version of OCaml.
val output_value : unit BatIO.output -> 'a -> unit
Write the representation of a structured value of any type to a channel. Circularities and sharing inside the value are detected and preserved. The object can be read back, by the function BatPervasives.input_value. See the description of module Marshal for more information. BatPervasives.output_value is equivalent to Marshal.output with an empty list of flags.
val close_out : unit BatIO.output -> unit
Close the given channel, flushing all buffered write operations. Output functions raise a Sys_error exception when they are applied to a closed output channel, except close_out and flush, which do nothing when applied to an already closed channel.
Raises Sys_error if the operating system signals an error when flushing or closing.
val close_out_noerr : unit BatIO.output -> unit
Same as close_out, but ignore all errors.

General input functions

val input_file : ?bin:bool -> string -> string
returns the data of a given filename.
val open_in : ?mode:BatFile.open_in_flag list ->
?perm:BatFile.permission -> string -> BatIO.input
Open the named file for reading. You will need to close the file once you have finished using it.

You may use optional argument mode to decide whether the opening should fail if the file doesn't exist yet (by default) or whether the file should be created if it doesn't exist yet, whether the opening should fail if the file already exists or not (by default), whether the file should be read as binary (by default) or as text, and whether reading should be non-blocking.

You may use optional argument perm to specify the permissions of the file, should it be created, as per Unix conventions. By default, files are created with default permissions (which depend on your setup).
Raises Sys_error if the file could not be opened.

val open_in_bin : string -> BatIO.input
Same as Pervasives.open_in, but the file is opened in binary mode, so that no translation takes place during reads. On operating systems that do not distinguish between text mode and binary mode, this function behaves like Pervasives.open_in.
val open_in_gen : Pervasives.open_flag list -> int -> string -> BatIO.input
Deprecated.Use open_in instead
open_in mode perm filename opens the named file for reading, as described above. The extra arguments mode and perm specify the opening mode and file permissions. Pervasives.open_in and Pervasives.open_in_bin are special cases of this function.
val input_char : BatIO.input -> char
Read one character from the given input channel.
Raises End_of_file if there are no more characters to read.
val input_line : BatIO.input -> string
Read characters from the given input channel, until a newline character is encountered. Return the string of all characters read, without the newline character at the end.
Raises End_of_file if the end of the file is reached at the beginning of line.
val input : BatIO.input -> string -> int -> int -> int
input ic buf pos len reads up to len characters from the given channel ic, storing them in string buf, starting at character number pos. It returns the actual number of characters read, between 0 and len (inclusive). A return value of 0 means that the end of file was reached. A return value between 0 and len exclusive means that not all requested len characters were read, either because no more characters were available at that time, or because the implementation found it convenient to do a partial read; input must be called again to read the remaining characters, if desired. (See also Pervasives.really_input for reading exactly len characters.)
Raises Invalid_argument if pos and len do not designate a valid substring of buf.
val really_input : BatIO.input -> string -> int -> int -> unit
really_input ic buf pos len reads len characters from channel ic, storing them in string buf, starting at character number pos.
Raises
val input_byte : BatIO.input -> int
Same as Pervasives.input_char, but return the 8-bit integer representing the character.
Raises End_of_file if an end of file was reached.
val input_binary_int : BatIO.input -> int
Read an integer encoded in binary format (4 bytes, big-endian) from the given input channel. See Pervasives.output_binary_int.
Raises End_of_file if an end of file was reached while reading the integer.
val input_binary_float : BatIO.input -> float
Read a float encoded in binary format (8 bytes, IEEE 754 double format) from the given input channel. See Pervasives.output_binary_float.
Raises End_of_file if an end of file was reached while reading the float.
val input_value : BatIO.input -> 'a
Read the representation of a structured value, as produced by BatPervasives.output_value, and return the corresponding value. This function is identical to Marshal.input; see the description of module Marshal for more information, in particular concerning the lack of type safety.
val close_in : BatIO.input -> unit
Close the given channel. Input functions raise a Sys_error exception when they are applied to a closed input channel, except close_in, which does nothing when applied to an already closed channel.
Raises Sys_error if the operating system signals an error.
val close_in_noerr : BatIO.input -> unit
Same as close_in, but ignore all errors.

Fundamental functions and operators

val identity : 'a -> 'a
The identity function.
val undefined : ?message:string -> 'a -> 'b
The undefined function.

Evaluating undefined x always fails and raises an exception "Undefined". Optional argument message permits the customization of the error message.

val (@@) : ('a -> 'b) -> 'a -> 'b

Function application. f @@ x is equivalent to f x. However, it binds less tightly (between :: and =,<,>,etc) and is right-associative, which makes it useful for composing sequences of function calls without too many parentheses. It is similar to Haskell's $. Note that it replaces pre-2.0 **> and <|.
val (%) : ('a -> 'b) -> ('c -> 'a) -> 'c -> 'b
Function composition: the mathematical o operator. f % g is fun x -> f (g x). It is similar to Haskell's ..

Examples: the following are equivalent: f (g (h x)), f @@ g @@ h x, f % g % h @@ x.

val (|>) : 'a -> ('a -> 'b) -> 'b
The "pipe": function application. x |> f is equivalent to f x.

This operator is commonly used to write a function composition by order of evaluation (the order used in object-oriented programming) rather than by inverse order (the order typically used in functional programming).

For instance, g (f x) means "apply f to x, then apply g to the result." The corresponding notation in most object-oriented programming languages would be somewhere along the lines of x.f.g.h(), or "starting from x, apply f, then apply g." In OCaml, using the ( |> ) operator, this is written x |> f |> g |> h.

This operator may also be useful for composing sequences of function calls without too many parentheses.

val (%>) : ('a -> 'b) -> ('b -> 'c) -> 'a -> 'c
Piping function composition. f %> g is fun x -> g (f x). Whereas f % g applies g first and f second, f %> g applies f, then g. Note that it plays well with pipes, so for instance x |> f %> g %> h |> i %> j yields the expected result... but in such cases it's still recommended to use |> only. Note that it replaces pre-2.0 |-, which didn't integrate with pipes.
val (|?) : 'a option -> 'a -> 'a
Like BatOption.default, with the arguments reversed. None |? 10 returns 10, while Some "foo" |? "bar" returns "foo".

Note This operator does not short circuit like || ) and && ). Both arguments will be evaluated.
Since 2.0

val flip : ('a -> 'b -> 'c) -> 'b -> 'a -> 'c
Argument flipping.

flip f x y is f y x. Don't abuse this function, it may shorten considerably your code but it also has the nasty habit of making it harder to read.

val curry : ('a * 'b -> 'c) -> 'a -> 'b -> 'c
Convert a function which accepts a pair of arguments into a function which accepts two arguments.

curry f is fun x y -> f (x,y)

val uncurry : ('a -> 'b -> 'c) -> 'a * 'b -> 'c
Convert a function which accepts a two arguments into a function which accepts a pair of arguments.

uncurry f is fun (x, y) -> f x y

val neg : ('a -> bool) -> 'a -> bool
neg p returns a new predicate that is the negation of the given predicate. That is, the new predicate returns false when the input predicate returns true and vice versa. This is for predicates with one argument.

neg p is fun x -> not (p x)

val neg2 : ('a -> 'b -> bool) -> 'a -> 'b -> bool
as neg but for predicates with two arguments
val const : 'a -> 'b -> 'a
Ignore its second argument.

const x is the function which always returns x.

val unique : unit -> int
Returns an unique identifier every time it is called.

Note This is thread-safe.

val tap : ('a -> unit) -> 'a -> 'a
Allows application of a function in the middle of a pipe sequence without disturbing the sequence. x |> tap f evaluates to x, but has the side effect of f x. Useful for debugging.
val finally : (unit -> unit) -> ('a -> 'b) -> 'a -> 'b
finally fend f x calls f x and then fend() even if f x raised an exception.
val with_dispose : dispose:('a -> unit) -> ('a -> 'b) -> 'a -> 'b
with_dispose dispose f x invokes f on x, calling dispose x when f terminates (either with a return value or an exception).
val forever : ('a -> 'b) -> 'a -> unit
forever f x invokes f on x repeatedly (until an exception occurs).
val ignore_exceptions : ('a -> 'b) -> 'a -> unit
ignore_exceptions f x invokes f on x, ignoring both the returned value and the exceptions that may be raised.
val verify_arg : bool -> string -> unit
verify_arg condition message will raise Invalid_argument message if condition is false, otherwise it does nothing.
Since 2.0
val args : unit -> string BatEnum.t
An enumeration of the arguments passed to this program through the command line.

args () is given by the elements of Sys.argv, minus the first element.

val exe : string
The name of the current executable.

exe is given by the first argument of Sys.argv


Enumerations

In OCaml Batteries Included, all data structures are enumerable, which means that they support a number of standard operations, transformations, etc. The general manner of enumerating the contents of a data structure is to invoke the enum function of your data structure.

For instance, you may use the BatPervasives.foreach loop to apply a function f to all the consecutive elements of a string s. For this purpose, you may write either foreach (String.enum s) f or open
   String in foreach (enum s) f
. Either possibility states that you are enumerating through a character string s. Should you prefer your enumeration to proceed from the end of the string to the beginning, you may replace String.enum with String.backwards. Therefore, either foreach (String.backwards s)
   f
or open String in foreach (backwards s) f will apply f to all the consecutive elements of string s, from the last to the first.

Similarly, you may use List.enum instead of String.enum to visit the elements of a list in the usual order, or List.backwards instead of String.backwards to visit them in the opposite order, or Hashtbl.enum for hash tables, etc.

More operations on enumerations are defined in module BatEnum, including the necessary constructors to make your own structures enumerable.

The various kinds of loops are detailed further in this documentation.

val foreach : 'a BatEnum.t -> ('a -> unit) -> unit
Imperative loop on an enumeration.

foreach e f applies function f to each successive element of e. For instance, foreach (1 -- 10) print_int invokes function print_int on 1, 2, ..., 10, printing 12345678910.

Note This function is one of the many loops available on enumerations. Other commonly used loops are BatPervasives.iter (same usage scenario as foreach, but with different notations), BatPervasives.map (convert an enumeration to another enumeration) or BatPervasives.fold (flatten an enumeration by applying an operation to each element).


General-purpose loops

The following functions are the three main general-purpose loops available in OCaml. By opposition to the loops available in imperative languages, OCaml loops are regular functions, which may be passed, composed, currified, etc. In particular, each of these loops may be considered either as a manner of applying a function to a data structure or as transforming a function into another function which will act on a whole data structure.

For instance, if f is a function operating on one value, you may lift this function to operate on all values of an enumeration (and consequently on all values of any data structure of OCaml Batteries Included) by applying BatPervasives.iter, BatPervasives.map or BatPervasives.fold to this function.

val iter : ('a -> unit) -> 'a BatEnum.t -> unit
Imperative loop on an enumeration. This loop is typically used to lift a function with an effect but no meaningful result and get it to work on enumerations.

If f is a function iter f is a function which behaves as f but acts upon enumerations rather than individual elements. As indicated in the type of iter, f must produce values of type unit (i.e. f has no meaningful result) the resulting function produces no meaningful result either.

In other words, iter f is a function which, when applied upon an enumeration e, calls f with each element of e in turn.

For instance, iter f (1 -- 10) invokes function f on 1, 2, ..., 10 and produces value ().

val map : ('a -> 'b) -> 'a BatEnum.t -> 'b BatEnum.t
Transformation loop on an enumeration, used to build an enumeration from another enumeration. This loop is typically used to transform an enumeration into another enumeration with the same number of elements, in the same order.

If f is a function, map f e is a function which behaves as f but acts upon enumerations rather than individual elements -- and builds a new enumeration from the results of each application.

In other words, map f is a function which, when applied upon an enumeration containing elements e0, e1, ..., produces enumeration f e0, f e1, ...

For instance, if odd is the function which returns true when applied to an odd number or false when applied to an even number, map odd (1 -- 10) produces enumeration true, false, true, ..., false.

Similarly, if square is the function fun x -> x * x, map square (1 -- 10) produces the enumeration of the square numbers of all numbers between 1 and 10.

val filter_map : ('a -> 'b option) -> 'a BatEnum.t -> 'b BatEnum.t
Similar to a map, except that you can skip over some items of the incoming enumeration by returning None instead of Some value. Think of it as a BatPervasives.filter combined with a BatPervasives.map.
val reduce : ('a -> 'a -> 'a) -> 'a BatEnum.t -> 'a
Transformation loop on an enumeration, used to build a single value from an enumeration.

If f is a function and e is an enumeration, reduce f e applies function f to the first two elements of e, then to the result of this expression and to the third element of e, then to the result of this new expression and to the fourth element of e...

In other words, reduce f e returns a0 if e contains only one element a0, otherwise f (... (f (f a0) a1) ...) aN where a0,a1..aN are the elements of e.
Raises Not_found if e is empty.

For instance, if add is the function fun x y -> x + y, reduce add is the function which computes the sum of the elements of an enumeration -- and doesn't work on empty enumerations. Therefore, reduce add (1 -- 10) produces result 55.

val fold : ('b -> 'a -> 'b) -> 'b -> 'a BatEnum.t -> 'b
Transformation loop on an enumeration, used to build a single value from an enumeration. This is the most powerful general-purpose loop and also the most complex.

If f is a function, fold f v e applies f v to the first element of e, then, calling acc_1 the result of this operation, applies f acc_1 to the second element of e, then, calling acc_2 the result of this operation, applies f acc_2 to the third element of e...

In other words, fold f v e returns v if e is empty, otherwise f (... (f (f v a0) a1) ...) aN where a0,a1..aN are the elements of e.

For instance, if add is the function fun x y -> x + y, fold add 0 is the function which computes the sum of the elements of an enumeration. Therefore, fold add 0 (1 -- 10) produces result 55.

val scanl : ('b -> 'a -> 'b) -> 'b -> 'a BatEnum.t -> 'b BatEnum.t
Functional loop on an enumeration, used to build an enumeration from both an enumeration and an initial value. This function may be seen as a variant of BatPervasives.fold which returns not only the final result of BatPervasives.fold but the enumeration of all the intermediate results of BatPervasives.fold.

If f is a function, scanl f v e is applies f v to the first element of e, then, calling acc_1 the result of this operation, applies f acc_1 to the second element of e, then, calling acc_2 the result of this operation, applies f acc_2 to the third element of e...

For instance, if add is the function fun x y -> x + y, scanl add 0 is the function which computes the sum of the elements of an enumeration. Therefore, scanl add 0 (1 -- 10) produces result the enumeration with elements 0, 1, 3, 6, 10,
    15, 21, 28, 36, 45, 55
.

val (/@) : 'a BatEnum.t -> ('a -> 'b) -> 'b BatEnum.t
val (@/) : ('a -> 'b) -> 'a BatEnum.t -> 'b BatEnum.t
Mapping operators.

These operators have the same meaning as function BatPervasives.map but are sometimes more readable than this function, when chaining several transformations in a row.

val (//@) : 'a BatEnum.t -> ('a -> 'b option) -> 'b BatEnum.t
val (@//) : ('a -> 'b option) -> 'a BatEnum.t -> 'b BatEnum.t
Map combined with filter. Same as BatPervasives.filter_map.

Other operations on enumerations

val exists : ('a -> bool) -> 'a BatEnum.t -> bool
exists f e returns true if there is some x in e such that f x
val for_all : ('a -> bool) -> 'a BatEnum.t -> bool
exists f e returns true if for every x in e, f x is true
val find : ('a -> bool) -> 'a BatEnum.t -> 'a
find f e returns the first element x of e such that f x returns true, consuming the enumeration up to and including the found element, or, raises Not_found if no such element exists in the enumeration, consuming the whole enumeration in the search.

Since find consumes a prefix of the enumeration, it can be used several times on the same enumeration to find the next element.
Raises Not_found if no element in the whole enumeration satisfies the predicate

val peek : 'a BatEnum.t -> 'a option
peek e returns None if e is empty or Some x where x is the next element of e. The element is not removed from the enumeration.
val get : 'a BatEnum.t -> 'a option
get e returns None if e is empty or Some x where x is the next element of e, in which case the element is removed from the enumeration.
val push : 'a BatEnum.t -> 'a -> unit
push e x will add x at the beginning of e.
val junk : 'a BatEnum.t -> unit
junk e removes the first element from the enumeration, if any.
val filter : ('a -> bool) -> 'a BatEnum.t -> 'a BatEnum.t
filter f e returns an enumeration over all elements x of e such as f x returns true.
val (//) : 'a BatEnum.t -> ('a -> bool) -> 'a BatEnum.t
Filtering (pronounce this operator name "such that").

For instance, (1 -- 37) // odd is the enumeration of all odd numbers between 1 and 37.

val concat : 'a BatEnum.t BatEnum.t -> 'a BatEnum.t
concat e returns an enumeration over all elements of all enumerations of e.
val (--) : int -> int -> int BatEnum.t
Enumerate numbers.

5 -- 10 is the enumeration 5,6,7,8,9,10. 10 -- 5 is the empty enumeration

val (--^) : int -> int -> int BatEnum.t
Enumerate numbers, without the right endpoint

5 -- 10 is the enumeration 5,6,7,8,9.

val (--.) : float * float -> float -> float BatEnum.t
(a, step) --. b) creates a float enumeration from a to b with an increment of step between elements.

(5.0, 1.0) --. 10.0 is the enumeration 5.0,6.0,7.0,8.0,9.0,10.0. (10.0, -1.0) --. 5.0 is the enumeration 10.0,9.0,8.0,7.0,6.0,5.0. (10.0, 1.0) --. 1.0 is the empty enumeration.

val (---) : int -> int -> int BatEnum.t
As --, but accepts enumerations in reverse order.

5 --- 10 is the enumeration 5,6,7,8,9,10. 10 --- 5 is the enumeration 10,9,8,7,6,5.

val (--~) : char -> char -> char BatEnum.t
As ( -- ), but for characters.
val print : ?first:string ->
?last:string ->
?sep:string ->
('a BatInnerIO.output -> 'b -> unit) ->
'a BatInnerIO.output -> 'b BatEnum.t -> unit
Print and consume the contents of an enumeration.

Results

type ('a, 'b) result = ('a, 'b) BatInnerPervasives.result = 
| Ok of 'a
| Bad of 'b (*
The result of a computation - either an Ok with the normal result or a Bad with some value (often an exception) containing failure information
*)
This type represents the outcome of a function which has the possibility of failure. Normal results of type 'a are marked with Ok, while failure values of type 'b are marked with Bad.

This is intended to be a safer alternative to functions raising exceptions to signal failure. It is safer in that the possibility of failure has to be handled before the result of that computation can be used.

For more functions related to this type, see the BatResult module.

val ignore_ok : ('a, exn) result -> unit
ignore_ok (f x) ignores the result of f x if it's ok, but throws the exception contained if Bad is returned.
val ok : ('a, exn) result -> 'a
f x |> ok unwraps the Ok result of f x and returns it, or throws the exception contained if Bad is returned.
val wrap : ('a -> 'b) -> 'a -> ('b, exn) result
wrap f x wraps a function that would normally throw an exception on failure such that it now returns a result with either the Ok return value or the Bad exception.

Thread-safety internals

Unless you are attempting to adapt Batteries Included to a new model of concurrency, you probably won't need this.

val lock : BatConcurrent.lock Pervasives.ref
A lock used to synchronize internal operations.

By default, this is BatConcurrent.nolock. However, if you're using a version of Batteries compiled in threaded mode, this uses BatMutex. If you're attempting to use Batteries with another concurrency model, set the lock appropriately.