# Module `Current_incr`

A simple library for incremental computations. Based on "Adaptive Functional Programming" https://www.cs.cmu.edu/~guyb/papers/popl02.pdf

The basic idea is:

- You run a computation, which produces a result.
- The library records information about the dependencies of the result.
- You can then change one or more inputs and it will rerun just the parts of the computation that are needed to update the result.

It is similar to Jane Street's "incremental" library, but much smaller and has no external dependencies.

It is also similar to "react", but the results do not depend on the behaviour of the garbage collector. In particular, functions stop being called as soon as they are no longer needed, and cannot be called with "impossible" inputs.

### Changeable values

`val const : 'a -> 'a t`

`const x`

is the constant value`x`

.

### Changeable computations

`type 'a cc`

An

`'a cc`

is a changeable computation that can be run to get a value of type`'a`

. Internally, it is a function that takes a destination variable, reads zero or more other changeable values, and then updates the destination.

`val read : 'a t -> ('a -> 'b cc) -> 'b cc`

`read x f`

is a computation that depends on`x`

.`f`

will be called as necessary to ensure that the result stays up-to-date with changes in`x`

.

`val write : ?eq:('a -> 'a -> bool) -> 'a -> 'a cc`

`write x`

is a computation that always returns`x`

.- parameter eq
If

`eq old_value x = true`

then the new value is considered equal and no update is done. The default is`(==)`

. It is always safe (though inefficient) to return`false`

here.

### External operations

These functions are used to interface between the changeable system and other systems (e.g. Lwt).

`type 'a var`

A mutable value that can be changed using

`change`

. Internally,`type 'a var = 'a t`

, but it's useful to distinguish between values that are inputs to the system and values that only change in response to computations being rerun.

`val var : 'a -> 'a var`

`var x`

creates a new variable initially set to`x`

.

`val change : ?eq:('a -> 'a -> bool) -> 'a var -> 'a -> unit`

`change x v`

sets the current value of`x`

to`v`

and adds anything that depends on it to the rebuild queue. You must call`propagate`

after this to update everything (you can change several things together and then call`propagate`

once). This function can only be used from the top-level, not from within a computation.

`val observe : 'a t -> 'a`

`observe t`

is the current value of`t`

. If`change`

has been used then the value may be stale until`propagate`

is called.