Module adapton::macros [−] [src]

Macros to make using the engine module's interface more ergonomic.

Adapton programming model

Adapton roles: Adapton proposes editor and achivist roles:
- Editor role: Creates and mutates input, observes (demands) output of incremental computations.
- Archivist role: Performs cached computations that consume incremental input and produce incremental output.

The examples below illustrate these roles, in increasing complexity.

Programming primitives: The following list of primitives covers the core features of the Adapton engine. Each primitive below is meaningful in each of the two, editor and archivist, roles:

  • Ref cell allocation: Mutable input (editor role), and cached data structures that change across runs (archivist role).
  • Observation and demand: Both editor and archivist role.
  • Thunk Allocation: Both editor and archivist role.
    • Thunk allocation, without demand:
    • Thunk allocation, with demand:
      • memo! -- Preferred version
      • let_memo! -- Useful in simple examples
      • eager! -- Deprecated; use memo! instead.

Implicit counter for naming cells

cell!(123) uses a global counter to choose a unique name to hold 123. Important note: This may be appopriate for the Editor role, but is never appropriate for the Archivist role.

let c : Art<usize> = cell!( 123 );

assert_eq!( get!(c), 123 );
assert_eq!( get!(c), force(&c) );

Explicitly-named cells

Sometimes we name a cell using a Rust identifier. We specify this case using the notation [ name ], which specifies that the cell's name is a string, constructed from the Rust identifer name:

let c : Art<usize> = cell!([c] 123);

assert_eq!(get!(c), 123);
assert_eq!(get!(c), force(&c));

Optionally-named cells

Most generally, we supply an expression optional_name of type Option<Name> to specify the name for the Art. This Art is created by either cell or put, in the case that optional_name is Some(name) or None, respectively:

let n : Name = name_of_str(stringify!(c));
let c : Art<usize> = cell!([Some(n)]? 123);

assert_eq!(get!(c), 123);
assert_eq!(get!(c), force(&c));

let c = cell!([None]? 123);

assert_eq!(get!(c), 123);
assert_eq!(get!(c), force(&c));

Demand-driven change propagation

The example below demonstrates demand-driven change propagation, which is unique to Adapton's approach to incremental computation. The example constructs two mutable inputs, nom and den, an intermediate subcomputation div that divides the numerator in nom by the denominator in den, and a thunk check that first checks whether the denominator is zero (returning zero if so) and if non-zero, returns the value of the division.

use adapton::macros::*;
use adapton::engine::*;
manage::init_dcg();

// Two mutable inputs, for numerator and denominator of division
let num = cell!(42); 
let den = cell!(2);

// In Rust, cloning is explicit:
let den2 = den.clone(); // clone _global reference_ to cell.
let den3 = den.clone(); // clone _global reference_ to cell, again.

// Two subcomputations: The division, and a check thunk with a conditional expression
let div   = thunk![ get!(num) / get!(den) ];
let check = thunk![ if get!(den2) == 0 { None } else { Some(get!(div)) } ];

// Observe output of `check` while we change the input `den`
// Step 1: (Explained in detail, below)
assert_eq!(get!(check), Some(21));

// Step 2: (Explained in detail, below)
set(&den3, 0);
assert_eq!(get!(check), None);

// Step 3: (Explained in detail, below)
set(&den3, 2);
assert_eq!(get!(check), Some(21));  // division is reused

The programmer's changes and observations in the last lines induce the following change propagation behavior:

  1. When the check is demanded the first time, it executes the condition, and den holds 2, which is non-zero. Hence, the else branch executes get!(div), which demands the output of the division, 21.

  2. After this first observation of check, the programmer changes den to 0, and re-demands the output of check. In response, change propagation first re-executes the condition (not the division), and the condition branches to the then branch, resulting in None; in particular, it does not re-demand the div node, though this node still exists in the DCG.

  3. Next, the programmer changes den back to its original value, 2, and re-demands the output of check. In response, change propagation re-executes the condition, which re-demands the output of div. Change propagation attempts to "clean" the div node before re-executing it. To do so, it compares its last observations of num and den to their current values, of 42 and 2, respectively. In so doing, it finds that these earlier observations match the current values. Consequently, it reuses the output of the division (21) without having to re-execute the division.

For a graphical illustration of this behavior, see these slides.

In the academic literature on Adapton, we refer to this three-step pattern as switching: The demand of div switches from being present (in step 1) to absent (in step 2) to present (in step 3).

Past work on self-adjusting computation does not support this switching pattern directly: Because of its change propagation semantics, it would "forget" the division in step 2, and rerun it from-scratch in step 3.

Furthermore, some other change propagation algorithms base their re-execution schedule on "node height" (of the graph's topological ordering). These algorithms may also have undesirable behavior. In particular, they may re-execute the division in step 2, though it is not presently in demand. For an example, see this gist.

Memoization

Memoization provides a mechanism for caching the results of subcomputations; it is a crtical feature of Adapton's approach to incremental computation.

In Adapton, each memoization point has three ingredients:

  • A function expression (of type Fn)

  • Zero or more arguments. Each argument type must have an implementation for the traits Eq + Clone + Hash + Debug. The traits Eq and Clone are both critical to Adapton's caching and change propagation engine. The trait Hash is required when Adapton's naming strategy is structural (e.g., where function names are based on the hashes of their arguments). The trait Debug is useful for debugging, and reflection.

  • An optional name, which identifies the function call for reuse later.

    • When this optional name is None, the memoization point may be treated in one of two ways: either as just an ordinary, uncached function call, or as a cached function call that is identified structurally, by its function pointer and arguments. Adapton permits structural subcomputations via the engine's structural function.

    • When this is Some(name), the memoization point uses name to identify the work performed by the function call, and its result. Critically, in future incremental runs, it is possible for name to associate with different functions and/or argument values.

Optional name version

The following form is preferred:

memo!( [ optional_name ]? fnexp ; lab1 : arg1, ..., labk : argk )

It accepts an optional name, of type Option<Name>, and an arbitrary function expression fnexp (closure or function pointer). Like the other forms, it requires that the programmer label each argument.

Example

let opnm  : Option<Name> = Some(name_unit());
let (t,z) : (Art<usize>, usize) = 
  memo!([opnm]?
    |x:usize,y:usize|{ if x > y { x } else { y }};
     x:10,   y:20   );

assert_eq!(z, 20);
assert_eq!(force(&t), 20);

Thunks

The following form is preferred:

thunk!( [ optional_name ]? fnexp ; lab1 : arg1, ..., labk : argk )

It accepts an optional name, of type Option<Name>, and an arbitrary function expression fnexp (closure or function pointer). Like the other forms, it requires that the programmer label each argument.

Example

let opnm  : Option<Name> = Some(name_unit());
let t : Art<usize> =
  thunk!([opnm]?
    |x:usize,y:usize|{ if x > y { x } else { y }};
     x:10,   y:20   );

assert_eq!(force(&t), 20);

Mapping observations makes them more precise

Suppose that we want to project only one field of type A from a pair within an Art<(A,B)>. If the field of type B changes, our observation of the A field will not be affected.

Below, we show that using force_map prunes the dirtying phase of change propagation. Doing so means that computations that would otherwise be dirty and cleaned via re-execution are never diritied in the first place. We show a simple example of projecting a pair.

To observe this fact, this test traces the engine, counts the number of dirtying steps, and ensures that this count is zero, as expected.

use adapton::macros::*;
use adapton::engine::*;
use adapton::reflect;
manage::init_dcg();    

// Trace the behavior of change propagation; ensure dirtying works as expected
reflect::dcg_reflect_begin();

let pair  = cell!((1234, 5678));
let pair1 = pair.clone();

let t = thunk![{
  // Project the first component of pair:
  let fst = force_map(&pair, |_,x| x.0); 
  fst + 100
}];

// The output is `1234 + 100` = `1334`
assert_eq!(force(&t), 1334);

// Update the second component of the pair; the first is still 1234
set(&pair1, (1234, 8765));

// The output is still `1234 + 100` = `1334`
assert_eq!(force(&t), 1334);

// Assert that nothing was dirtied (due to using `force_map`)
let traces = reflect::dcg_reflect_end();
let counts = reflect::trace::trace_count(&traces, None);
assert_eq!(counts.dirty.0, 0);
assert_eq!(counts.dirty.1, 0);

Nominal memoization: Toy Examples

Adapton offers nominal memoization, which uses first-class names (each of type Name) to identify cached computations and data. Behind the scenes, these names control how and when the engine overwrites cached data and computations. As such, they permit patterns of programmatic cache eviction.

For a simple illustration, we memoize several function calls to sum with different names and arguments. In real applications, the memoized function typically performs more work than summing two machine words. :)

use adapton::macros::*;
use adapton::engine::*;
use adapton::reflect;

// create an empty DCG (demanded computation graph)
manage::init_dcg();

// a simple function (memoized below for illustration purposes;
// probably actually not worth it!)
fn sum(x:usize, y:usize) -> usize {
    x + y
}

// Optional: Traces what the engine does below (for diagnostics, testing, illustration)
reflect::dcg_reflect_begin();

let nm_a_0 : Name = name_of_str("a"); // name "a"
let nm_a_1 : Name = name_of_str("a"); // name "a" (another copy)
let nm_b_0 : Name = name_of_str("b"); // name "b"
let nm_b_1 : Name = name_of_str("b"); // name "b" (another copy)

// create a memo entry, named "a", that remembers that `sum(42,43) = 85`
let res1 : usize = memo!(nm_a_0 =>> sum, x:42, y:43);

// same name "a", same arguments (42, 43) => reuses the memo entry above for `res1`
let res2 : usize = memo!(nm_a_1 =>> sum, x:42, y:43);

// different name "b", same arguments (42, 43) => will *not* match `res1`; creates a new entry
let res3 : usize = memo!(nm_b_0 =>> sum, x:42, y:43);

// same name "b", different arguments; will *overwrite* entry named "b" with new args & result
let res4 : usize = memo!(nm_b_1 =>> sum, x:55, y:66);

// Optional: Assert what happened above, in terms of analytical counts
let traces = reflect::dcg_reflect_end();
let counts = reflect::trace::trace_count(&traces, None);

// Editor allocated two thunks ("a" and "b")
assert_eq!(counts.alloc_fresh.0, 2);

// Editor allocated one thunk without changing it ("a", with same args)
assert_eq!(counts.alloc_nochange.0, 1);

// Editor allocated one thunk by changing it ("b", different args)
assert_eq!(counts.alloc_change.0, 1);

// Archivist allocated nothing
assert_eq!(counts.alloc_fresh.1, 0);

Some notes about the code above:

  • Callsite argument names: The macro memo! relies on programmer-supplied variable names in its macro expansion of these call sites, shown as x and y in the uses above. These can be chosen arbitrarily: So long as these symbols are distinct from one another, they can be any symbols, and need not actually match the formal argument names.

  • Type arguments: If the function call expects type arguments, memo! accomodates these calls with alternative syntax.

  • Spurious arguments: If the function call expects some later arguments that do not implement Eq, but are functionally determined by earlier ones that do (including the supplied Name), memo! accomodates these calls with alternative syntax. We call these arguments "spurious", since the Adapton engine does not check their identity when performing change propagation. Common examples include function values (e.g., anonymous closures).

Nominal Firewalls

This example demonstrates how nominal allocation mixes dirtying and cleaning behind the scenes: when the input changes, dirtying proceeds incrementally through the edges of the DCG, during cleaning. In some situations (Run 2, below), nominal allocation prevents dirtying from cascading, leading to finer-grained dependency tracking, and more incremental reuse. One might call this design pattern "nominal firewalls" (thanks to @nikomatsakis for suggesting the term "firewall" in this context).

First, consider this DCG:

//   cell                           +---- Legend ------------------+
//   a                              | [ 2 ]   ref cell holding 2   |
//   [ 2 ]                          |  (g)    thunk named 'g'      |
//     ^                            | ---->   force/observe edge   |
//     | force                      | --->>   allocation edge      |              
//     | 2                          +------------------------------+
//     |
//     |                 cell                                   cell
//     |    alloc 4       b      force 4           alloc 4       c
//    (g)------------->>[ 4 ]<--------------(h)-------------->>[ 4 ]
//     ^                                     ^
//     | force                               | force h,
//     | returns b                           | returns c
//     |                                     |
//    (f)------------------------------------+
//     ^
//     | force f,
//     | returns cell c
//     |
//  (root of demand)

In this graph, the ref cell b acts as the "firewall".

Below, we show a particular input change for cell a where a subcomputation h is never dirtied nor cleaned by change propagation (input change 2 to -2). We show another change to the same input where this subcomputation h is eventually dirtied and cleaned by Adapton, though not immediately (input change -2 to 3).

Here's the Rust code for generating this DCG, and these changes to its input cell, named "a":

use adapton::macros::*;
use adapton::engine::*;

fn demand_graph(a: Art<i32>) -> Art<i32> {
    let_memo!{
      c =(f)= { let a = a.clone();
        let_memo!{
          b =(g)={ let x = get!(a);
                   cell!([b] x * x) };
          c =(h)={ let x = get!(b); 
                   cell!([c] if x < 100 { x } else { 100 }) };
          c }};
      c }
}

manage::init_dcg();

// 1. Initialize input cell "a" to hold 2, and do the computation illustrated above:
let c = demand_graph(let_cell!{a = 2; a});

// 2. Change input cell "a" to hold -2, and do the computation illustrated above:
let c = demand_graph(let_cell!{a = -2; a});

// 3. Change input cell "a" to hold 3, and do the computation illustrated above:
let c = demand_graph(let_cell!{a = 3; a});

The let_memo! macro above expands as follows:

fn demand_graph__mid_macro_expansion(a: Art<i32>) -> Art<i32> {
    let f = let_thunk!{f = {
              let a = a.clone();
              let g = let_thunk!{g = {let x = get!(a);
                                      cell!([b] x * x)};
                                 g };
              let b = force(&g);
              let h = let_thunk!{h = {let x = get!(b); 
                                      cell!([c] if x < 100 { x } else { 100 })};
                                 h };
              let c = force(&h);
              c };
            f };
    let c = force(&f);
    c
};

In this example DCG, thunk f allocates and forces two sub-computations, thunks g and h. The first observes the input a and produces an intermediate result (ref cell b); the second observes this intermediate result and produces a final result (ref cell c), which both thunks h and f return as their final result.

Run 1. In the first computation, the input cell a holds 2, and the final resulting cell c holds 4.

Run 2. When the input cell a changes, e.g., from 2 to -2, thunks f and g are dirtied. Thunk g is dirty because it observes the changed input. Thunk f is dirty because it demanded (observed) the output of thunk g in the extent of its own computation.

Importantly, thunk h is not immediately dirtied when cell a changes. In a sense, cell a is an indirect ("transitive") input to thunk h. This fact may suggest that when cell a is changed from 2 to -2, we should dirty thunk h immediately. However, thunk h is related to this input only by reading a different ref cell (ref cell b) that depends, indirectly, on cell a, via the behavior of thunk g, on which thunk h does not directly depend: thunk h does not force thunk g.

Rather, when thunk f is re-demanded, Adapton will necessarily perform a cleaning process (aka, "change propagation"), re-executing g, its immediate dependent, which is dirty. Since thunk g merely squares its input, and 2 and -2 both square to 4, the output of thunk g will not change in this case. Consequently, the observers of cell b, which holds this output, will not be dirtied or re-executed. In this case, thunk h is this observer. In situations like these, Adapton's dirtying + cleaning algorithms do not dirty nor clean thunk h.

In sum, under this change, after f is re-demanded, the cleaning process will first re-execute g, the immediate observer of cell a. Thunk g will again allocate cell b to hold 4, the same value as before. It also yields this same cell pointer (to cell b). Consequently, thunk f is not re-executed, and is cleaned. Meanwhile, the outgoing (dependency) edges thunk of h are never dirtied.

Run 3. For some other change, e.g., from 2 to 3, thunk h would eventually be dirtied and cleaned.