package bam

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Module defining trees which is the main data-structure behind shrinking used by the Gen. This module shoul be used only when defining a new runner for a new Test framework.


This module encodes a non-bounded branching tree. Values are generated lazily via the module Seq.

type 'a t

Abstract representation of a tree.

type 'a tree = 'a t

Convenient alias if 'a t is shadowed.

val make : 'a -> ('a -> 'a Seq.t) -> 'a t

make f root returns a tree with root root and the children are recursively produced (lazily) with f.

val of_seq : 'a Seq.t -> 'a -> 'a t

of_seq seq root returns a tree whose root is root and the children are generated via seq. Hence, if seq is not empty, the depth of the tree is 1.

val root : 'a t -> 'a

root tree returns the root of the tree.

val map : ('a -> 'b) -> 'a t -> 'b t

map f tree maps f to all the elements of the tree. The merge function is reset to Seq.append. There is no automatic way to preserve the merge behavior with map. However, you can save the previous behavior with get_merge and set a new behavior with with_merge.

Searching functions as trees

binary_search ~initial ~origin implements a binary search enumerating elements. Elements are guaranteed to be in the interval origin;initial.

Note: If initial < origin, it returns a tree with the single node origin.

fractional_search ?exhaustive_search_digits ?precision_digits ~initial ~origin () returns a tree of depth one where the root is initial and children are ordered by the prefix ordering (modulo float representations) starting with origin. The children are always float between origin and initial starting with floats with few digits.

linear_search returns a tree of depth 1 whose root is initial and children are the number from origin included to initial excluded.

A monadic interface

val return : 'a -> 'a t

return value returns a tree containing a single node with value.

val bind : 'a t -> ('a -> 'b t) -> 'b t

bind tree f returns a tree where f is applied (lazily) to all the values of tree. Since f returns itself a tree, bind must be able to merge values of tree with the ones produced by f. This can be done via the merging process specified by the tree returned by f.

module Syntax : sig ... end

Merging values

By using bind, one needs to be able to merge sequences of trees. To understand why, let's give an informal definition of bind:

let rec bind tree f =
   let {root;children=left} = tree in
   (* children : 'a t Seq.t *)
   let {root; children=right} f root in
   (* How should we combine the sequence of trees denotes by [left] and [right]? *)

A natural way to merging them as denoted by the variable names is to use Seq.append. However, this is an arbitrary choice. Libraries using this module may redefine the default merging procedure to enable more complex behaviors.

Even though Seq.append is arbitrary, in practice it leads to predictable and easy to unerstand behaviors.

Notice that in the definition above, there are two trees, hence maybe two different merging behaviors. However, by typing, only one is allowed: the one resulting of the application of f root.

The type of the merge prevents 'a t to be an applicative functor.

val with_merge : merge:('a t Seq.t -> 'a t Seq.t -> 'a t Seq.t) -> 'a t -> 'a t

with_merge ~merge tree sets the merging behavior as merge.

val get_merge : 'a t -> 'a t Seq.t -> 'a t Seq.t -> 'a t Seq.t

get_merge tree returns the merge function for this tree.


module Forest : sig ... end

A forest can be considered as a non-empty sequence of trees. The functions declared in this module transposed naturally the functions provided on tree.


val shrink : ('a -> ('ok, 'err) Result.t) -> 'a t -> 'a

shrink tree f returns a value a that has the following specification:

  • f a = Error _
  • path(a,tree.root) = true
  • forall v, if v \in path(a, tree.root) then f v = Error _
  • if v' = Left(v) and v \in path(a, tree.root) then f v = Ok _

Assuming that f tree.root is Error _.

val crunch : int -> 'a t -> 'a t

crunch i tree returns a tree where the row i is merged with row i-1. Hence, when i <= 1, this is the identity function.


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