Module Bindlib

module Bindlib: sig .. end

The Bindlib library provides support for free and bound variables in the OCaml language. The main application is the construction of abstract types containing a binding structure (e.g., abstract syntax trees).

Author(s): Christophe Raffalli, Rodolphe Lepigre

Variables, binders and substitution

The Bindlib library provides two type constructors for building abstract syntax trees: 'a var and ('a,'b) binder. Intuitively, 'a var will be a representation for a free variable of type 'a, and ('a,'b) binder a represention for a term of type 'b depdening on a variable (or value) of type 'a (the type ('a,'b) binder can be seen as 'a -> 'b). Note that types 'a mvar and ('a,'b) mbinder are provided for handling arrays of variables.

type 'a var

Type of a free variable of type 'a.

type 'a mvar = 'a var array

Type of an array of variables of type 'a.

type (-'a, +'b) binder

Type of a binder for an element of type 'a into an element of type 'b. In terms of higher-order abstract syntax, it can be seen as 'a -> 'b.

type ('a, 'b) mbinder

Type of a binder for an array of elements of type 'a into an element of type 'b.

As an example, we give bellow the definition of a simple representation of the terms of the lambda-calculus.

    type term =
      | Var of term var
      | Abs of (term, term) binder
      | App of term * term

val subst : ('a, 'b) binder -> 'a -> 'b

subst b v substitutes (using application) the variable bound by b with the value b. This is a very efficient operations.

val msubst : ('a, 'b) mbinder -> 'a array -> 'b

msubst b vs substitutes (using application) the array of variables bound by b with the values vs. This is a very efficient operations. However, the length of the vs array should match the arity of the binder (see the function mbinder_arity).

Comming back to our lambda-calculus example, we can define the evaluation of a lambda-term as a simple recursive function using subst.

    let rec eval : term -> term = fun t ->
      match t with
      | App(f,a) ->
          begin
            match eval f with
            | Abs(b) -> eval (subst b a)
            | _      -> t
          end
      | _        -> t

val new_var : ('a var -> 'a) -> string -> 'a var

new_var mkfree name creates a new variable using a function mkfree and a name. The mkfree function is used to inject variables in the type of the corresponding elements. It is a form of syntactic wrapper.

val new_mvar : ('a var -> 'a) -> string array -> 'a mvar

new_mvar mkfree names creates a new array of variables using a function mkfree (see new_var) and a name.

Following on our example of the lambda-calculus, the mkfree function for variables of type term var could be defined as follows.

    let mkfree : term var -> term = fun x -> Var(x)

val name_of : 'a var -> string

name_of x returns a printable name for variable x.

val unbind : ('a var -> 'a) -> ('a, 'b) binder -> 'a var * 'b

unbind mkfree b breaks down the binder b into a variable, and the term in which this variable is now free. Note that the usual mkfree function is required, since unbind needs to create a new variable (its name will be that of the previously bound variable).

val unmbind : ('a var -> 'a) -> ('a, 'b) mbinder -> 'a mvar * 'b

unmbind mkfree b breaks down the binder b into variables, and the term in which these variables are now free. Again, the usual mkfree function is required, and the name of the new variables is based on that of all the variables that were previously bound.

An usual use of unbind is the wrinting of pretty-printing functions. The function given bellow transforms a lambda-term into a string. Note that the name_of function is used for variables.

    let rec to_string : term -> string = fun t ->
      match t with
      | Var(x)   -> name_of x
      | Abs(b)   -> let (x,t) = unbind mkfree b in
                    "\\" ^ name_of x ^ "." ^ to_string t
      | App(t,u) -> "(" ^ to_string t ^ ") " ^ to_string u

Constructing terms and binders in the bindbox

One of the main design priciple of the Bindlib library is efficiency. To obtain very fast substitutions, a price is paid at the construction of the terms. Indeed, binders (i.e., element of type ('a,'b) binder) cannot be defined directly. Instead, they are put together in the type 'a bindbox. It correspond to a term of type 'a with free variables that may still be bound in the future.

type +'a bindbox

Type of a term of type 'a under construction. Using this representation, the free variable of the term can be bound easily.

val box_of_var : 'a var -> 'a bindbox

box_of_var x builds a 'a bindbox from the 'a var x.

val box : 'a -> 'a bindbox

box e puts the value e into the 'a bindbox type, assuming that it is closed. Thus, if e contains variables, then they will not be considered free. This means that no variable of e will be available for binding.

val apply_box : ('a -> 'b) bindbox -> 'a bindbox -> 'b bindbox

apply_box bf ba applies the boxed function bf to a boxed argument ba inside the 'a bindbox type. This function is useful for constructing new expressions by applying a function with free variables to an argument with free variables. Note that the 'a bindbox type is an applicative functor. Its application operator (sometimes written "<*>") is apply_box, and its unit (sometimes called "pure") is box.

val box_apply : ('a -> 'b) -> 'a bindbox -> 'b bindbox

box_apply f ba applies the function f to a boxed argument ba. It is equivalent to apply_box (box f) ba, but is more efficient.

val box_apply2 : ('a -> 'b -> 'c) ->
       'a bindbox -> 'b bindbox -> 'c bindbox

box_apply2 f ba bb applies the function f to two boxed arguments ba and bb. It is equivalent to apply_box (apply_box (box f) ba) bb but it is more efficient.

val bind : ('a var -> 'a) ->
       string ->
       ('a bindbox -> 'b bindbox) ->
       ('a, 'b) binder bindbox

bind mkfree name f constructs a boxed binder from the function f. Note that name and mkfree are required to build the bound variable.

val mbind : ('a var -> 'a) ->
       string array ->
       ('a bindbox array -> 'b bindbox) ->
       ('a, 'b) mbinder bindbox

mbind mkfree names f constructs a boxed binder from function f, using mkfree and names to build the bound variables.

As mentioned earlier, terms with bound variables can only be build in the 'a bindbox type. To ease the writing of terms, it is a good practice to define "smart constructors" at the 'a bindbox level. Coming back to our lambda-calculus example, we can give the following smart constructors.

    let var : term var -> term bindbox =
      fun x -> box_of_var x

    let abs : string -> (term bindbox -> term bindbox) -> term bindbox =
      fun x f -> box_apply (fun b -> Abs(b)) (bind mkfree x f)

    let app : term bindbox -> term bindbox -> term bindbox =
      fun t u -> box_apply2 (fun t u -> App(t,u)) t u

val unbox : 'a bindbox -> 'a

unbox e can be called when the construction of a term is finished (e.g., when the desired variables have all been bound).

We can then easily define terms of the lambda-calculus as follows.

    let id    : term = (* \x.x *)
      unbox (abs "x" (fun x -> x))

    let fst   : term = (* \x.\y.x *)
      unbox (abs "x" (fun x -> abs "y" (fun _ -> x)))

    let omega : term = (* (\x.(x) x) \x.(x) x *)
      let delta = abs "x" (fun x -> app x x) in
      unbox (app delta delta)

val vbind : ('a var -> 'a) ->
       string ->
       ('a var -> 'b bindbox) ->
       ('a, 'b) binder bindbox

vbind mkfree name f constructs a boxed binder from the function f, as the bind function does. The difference here is that the domain of f is 'a var, and not 'a bindbox.

val mvbind : ('a var -> 'a) ->
       string array ->
       ('a var array -> 'b bindbox) ->
       ('a, 'b) mbinder bindbox

mvbind mkfree names f constructs a boxed binder from the function f as the mbind function does. However, the domain of f is 'a var, and not 'a bindbox.

Using the vbind function instead of the bind function, we can give an alternative smart constructor for lambda-abstraction. Note that we need to use the box_of_var to use a variable when defining a term.

    let abs_var : string -> (term var -> term bindbox) -> term bindbox =
      fun x f -> box_apply (fun b -> Abs(b)) (vbind mkfree x f)

    let delta : term = (* \x.(x) x *)
      unbox (abs_var "x" (fun x -> app (var x) (var x)))

val bind_var : 'a var ->
       'b bindbox -> ('a, 'b) binder bindbox

bind_var x b binds the variable x in b to produce a boxed binder. In fact, is used to implement bind and vbind.

val bind_mvar : 'a mvar ->
       'b bindbox -> ('a, 'b) mbinder bindbox

bind_mvar xs b binds the variables of xs in b to get a boxed binder. In fact, bind_mvar is used to implement mbind and mvbind.

More bindbox manipulation functions

In general, it is not difficult to use the box and apply_box functions to manipulate any kind of data in the 'a bindbox type. However, working with these functions alone can be tedious. The functions provided here can be used to manipulate standard data types in an optimised way.

val box_opt : 'a bindbox option -> 'a option bindbox

box_opt bo shifts the option type of bo into the bindbox.

val box_list : 'a bindbox list -> 'a list bindbox

box_list bs shifts the list type of bs into the bindbox.

val box_rev_list : 'a bindbox list -> 'a list bindbox

box_rev_list bs shifts the list type of bs into the bindbox, while reversing the list (for efficiency).

val box_array : 'a bindbox array -> 'a array bindbox

box_array bs shifts the array type of bs into the bindbox.

val box_apply3 : ('a -> 'b -> 'c -> 'd) ->
       'a bindbox ->
       'b bindbox -> 'c bindbox -> 'd bindbox

box_apply3 is similar to box_apply2.

val box_apply4 : ('a -> 'b -> 'c -> 'd -> 'e) ->
       'a bindbox ->
       'b bindbox ->
       'c bindbox -> 'd bindbox -> 'e bindbox

box_apply4 is similar to box_apply2 and box_apply3.

val box_pair : 'a bindbox -> 'b bindbox -> ('a * 'b) bindbox

box_pair ba bb is the same as box_apply2 (fun a b -> (a,b)) ba bb.

val box_triple : 'a bindbox ->
       'b bindbox -> 'c bindbox -> ('a * 'b * 'c) bindbox

box_trible is similar to box_pair, but for triples.

module type Map = sig .. end

Type of a module equipped with a map function.

module Lift (M : Map) : sig .. end

Functorial interface used to build lifting functions (i.e., functions that permute the 'a bindbox type with another type constructor) for any type equipped with a map function.

module type Map2 = sig .. end

Type of a module equipped with a "binary" map function.

module Lift2 (M : Map2) : sig .. end

Similar to the Lift functor, but handles "binary" map functions.

Attributes of variables and utilities

val prefix_of : 'a var -> string

prefix_of x returns the string prefix of variable x's name, which is only the first part of its full name (obtained with name_of x). The full name also contain an int suffix, which is defined as the longest suffix of digits in the full name.

val suffix_of : 'a var -> int

suffix_of x returns the int suffix of variable x's name. It consists in the longest digit suffix in the full name of x (see prefix_of).

val free_of : 'a var -> 'a

free_of x injects variable x into the corresponding type, relying on the mkfree function provided at the creation of the variable.

val hash_var : 'a var -> int

hash_var x computes a hash for variable x. Note that this function can be used with the Hashtbl module.

val compare_vars : 'a var -> 'b var -> int

compare_vars x y safely compares x and y. Note that it is unsafe to compare variables with Pervasive.compare.

val eq_vars : 'a var -> 'b var -> bool

eq_vars x y safely computes the equality of x and y. Note that it is unsafe to compare variables with the polymorphic equality function.

val copy_var : 'b var -> string -> ('a var -> 'a) -> 'a var

copy_var x name mkfree makes a copy of variable x, with a potentially different name and mkfree function. However, the copy is treated exactly as the original in terms of binding and substitution.

Attributes of binders and utilities

val binder_name : ('a, 'b) binder -> string

binder_name returns the name of the variable bound by the binder.

val binder_occur : ('a, 'b) binder -> bool

binder_occur b tests whether the bound variable occurs in b.

val binder_constant : ('a, 'b) binder -> bool

binder_constant b tests whether the binder b is constant (i.e., its bound variable does not occur).

val binder_closed : ('a, 'b) binder -> bool

binder_closed b test whether the binder b is closed (i.e., does not contain any free variable).

val binder_rank : ('a, 'b) binder -> int

binder_rank b gives the number of free variables contained in b.

val binder_compose_left : ('a -> 'b) -> ('b, 'c) binder -> ('a, 'c) binder

binder_compose_left f b precomposes the binder b with the function f without changing anything at the binding structure.

val binder_compose_right : ('a, 'b) binder -> ('b -> 'c) -> ('a, 'c) binder

binder_compose_rigth b f postcomposes the binder b with the function f without changing anything at the binding structure.

val mbinder_arity : ('a, 'b) mbinder -> int

mbinder_arity b gives the arity of the mbinder.

val mbinder_names : ('a, 'b) mbinder -> string array

mbinder_names b return the array of the names of the variables bound by the mbinder b.

val mbinder_occurs : ('a, 'b) mbinder -> bool array

mbinder_occurs b returns an array of bool indicating if the variables that are bound occur (i.e., are used).

val mbinder_constant : ('a, 'b) mbinder -> bool

mbinder_constant b indicates whether the mbinder b is constant. This means that none of its variables are used.

val mbinder_closed : ('a, 'b) mbinder -> bool

mbinder_closed b indicates whether b is closed.

val mbinder_rank : ('a, 'b) mbinder -> int

Attributes of bindboxes and utilities

val is_closed : 'a bindbox -> bool

is_closed b checks whether the bindbox b is closed.

val occur : 'a var -> 'b bindbox -> bool

occur x b tells whether variable x occurs in the bindbox b.

val bind_apply : ('a, 'b) binder bindbox ->
       'a bindbox -> 'b bindbox

bind_apply bb barg substitute the boxed binder bb with the boxed value barb in the bindbox type. This is useful for working with higher-order variables, which may be represented as binders themselves.

val mbind_apply : ('a, 'b) mbinder bindbox ->
       'a array bindbox -> 'b bindbox

mbind_apply bb bargs substitute the boxed binder bb with the boxed array of values barbs in the bindbox type. This is useful for working with higher-order variables.

val fixpoint : (('a, 'b) binder, ('a, 'b) binder) binder
       bindbox -> ('a, 'b) binder bindbox

fixpoint bb constructs a binder fixpoint. This very advanced feature can be used to build recursive definitions (like with OCaml's "let rec").

Working in a context

It is sometimes convenient to work in a context for variables, for example when one wishes to reserve variable names. The Bindlib library provides a type of contexts, together with functions for creating variables and for binding variables in a context.

type ctxt

Type of a context.

val empty_ctxt : ctxt

empty_ctxt denotes the empty context.

val new_var_in : ctxt ->
       ('a var -> 'a) -> string -> 'a var * ctxt

new_var_in ctxt mkfree name is similar to new_var mkfree name, but the variable names is chosen not to collide with the context ctxt. Note that the context that is returned contains the new variable name.

val new_mvar_in : ctxt ->
       ('a var -> 'a) -> string array -> 'a mvar * ctxt

new_mvar_in ctxt mkfree names is similar to new_mvar mkfree names, but it handles the context (see new_var_in).

val unbind_in : ctxt ->
       ('a var -> 'a) ->
       ('a, 'b) binder -> 'a var * 'b * ctxt

unbind_in ctxt mkfree b is similar to unbind mkfree b, but it handles the context (see new_mvar_in).

val unmbind_in : ctxt ->
       ('a var -> 'a) ->
       ('a, 'b) mbinder -> 'a mvar * 'b * ctxt

munbind_in ctxt mkfree b is like munbind mkfree b, but it handles the context (see new_mvar_in).

val bind_in : ctxt ->
       ('a var -> 'a) ->
       string ->
       ('a bindbox -> ctxt -> 'b bindbox) ->
       ('a, 'b) binder bindbox

bind_in ctxt mkfree name f is like bind mkfree name f, but it handles the context.

val mbind_in : ctxt ->
       ('a var -> 'a) ->
       string array ->
       ('a bindbox array -> ctxt -> 'b bindbox) ->
       ('a, 'b) mbinder bindbox

mbind_in ctxt mkfree names f is similar to mbind mkfree names f, but it handles the context.

Unsafe, advanced features

val reset_counter : unit -> unit

reset_counter () resets the unique identifier counter on which Bindlib relies. This function should only be called when previously generated data (e.g., variables) cannot be accessed anymore.

val dummy_bindbox : 'a bindbox

dummy_bindbox can be used for initialising structires (e.g., arrays). If unbox is called on a data structure containing a dummy_bindbox, the exception Failure "Invalid use of dummy_bindbox" is raised.

val binder_from_fun : string -> ('a -> 'b) -> ('a, 'b) binder

binder_from_fun name f transform a function into a binder. Note that the function is only called when the binder is substituted (see subst). This is not the recommended way of building binders. Nonetheless, it is useful for simple tasks such as contracting two binders into one, without copying the whole structure (e.g., transform \x.\y.t(x,y) into \x.t(x,x)).

val mbinder_from_fun : string array -> ('a array -> 'b) -> ('a, 'b) mbinder

mbinder_from_fun is similar to binder_from_fun.