Saturday 4 May 2019

The essence of (im)pure computation

This blog post is an informal introduction to a forthcoming research paper in collaboration with my students Koko Muroya and Todd Waugh Ambridge. This is the second post of dedicated to the SPARTAN calculus. The first post was How to make a programming language out of an algebra.

For the impatient reader I will summarise the point of the previous post:
algebra + thunking + copying = pure functional programming
The "algebra" part are the operations of the language. They include not only what we commonly think of as operations, such as arithmetic, but also function definition and application or recursion. The "thunking" is a way of postponing computations and "copying" is self explanatory. Together, thunking and copying are a means of providing control over the resources used in the process of computation, both time and space. A program that illustrates all these concepts put together is the factorial, which you can explore using the Spartan online visualiser.

How about "impure" features of programming languages? For example state and assignment. The first step is to add a new feature to our core calculus: sharing. In pure computation variables are "referentially transparent", which is a fancy way of saying that any term bind x to u in v can be equivalently rewritten by substituting u for x in v. This means that assignment cannot be naturally realised as an operation unless we add a notion of name or reference to the Spartan infrastructure:
new a = u in v
Contrast this with variable binding. The expression bind x to 0 in ASG(x, ADD(DEREF(x), 1)), which is Spartan syntax for something like int x=0; x=x+1, should be equivalent by referential transparency with 0=0; 0=0+1, which is clearly nonsense. On the other hand names are not copied, so new a = 0 in ASG(a, ADD(DEREF(a), 1)) is not subject to any substitutional simplification. To emphasise the fact that names are "like constants" rather than "like variables" we use a, b, ... to denote them.

In terms of diagrammatic representation, names (new) are represented as a new kind of node, an undecorated dot. Here are the two terms mentioned above, diagrammatically:


The (I)-labelled nodes play no active role, they are merely "adaptors", like between an European and an American socket. The first one is not a well-structured program and will block at run-time when it reaches this stage, which corresponds to ASG(0, ADD(DEREF(0), 1)):

The lighting bolt indicates that DEREF(0) (written as !0) is about to be evaluated -- but there is no rule for that.

On the other hand, the key stages for the second term are:
 to  to  to  to 

The evaluation of dereferencing, the (!) node, resolves in the argument of (+) being linked to whatever the (unlabelled) name node is linked to, namely the constant 0, which is duly copied. The addition (+) is computed in the obvious way. Finally, the assignment itself produces a dummy value (), and changes the term linked to by the name node to (1). At the same time this node becomes unreachable, i.e. "garbage".

The name can connect to anything, data ("reference") or code ("label"), storing functions or reference points in a program. The only feature of the name is that it is never copied, always shared. We can thus conclude that
Pure languages copy, impure languages share. Copying is done through variables, sharing is done through names.  
The issue of names and sharing is foundationally interesting, but not essential as far as expressiveness goes. Indeed, stateful-like behaviour can be encoded into (pure) higher-order functions using (for example) monads. Why do we then bother with adding native or primitive state, as we do? Simply because of the cost involved. The overhead required by encoding is significant.

In a language such as OCaml, which has both native state and higher-order functions, we can experiment by running the same computation both natively and using the state monad. On the computer I am writing this blog now the overhead of the simulated monadic state is approximately 3,200%. But this is contingent on many factors, which usually make benchmarks unreliable for such analyses.

In contrast, an abstract machine allows an abstract, platform-independent evaluation of performance by counting steps and associating costs with the various actions performed at each particular step. In the case of Spartan, the monadic simulation takes 3,498 steps (see Example 10) whereas the native state implementation takes 76 steps (see Example 9). The 46X overhead exhibited by the abstract machine is surprisingly close to the 32X overhead observed in the benchmark. Part of the execution (monadic simulation takes too long...) is shown in this side-by-side video:

The Spartan framework is flexible and general enough that we could express any known operations we could think of. Here is for example what can happen when you have a language combining call/cc, a very powerful control operator, with state. Such a combination is called "stored continuation" and it is so expressive that you can, for example, evaluate both branches of an if statement!. The code (see Example 6 for proper Spartan syntax and simulation) in generic functional syntax is:
let state = () in    
var saveState = λx.callcc(λc.c:=state; x) in    
let loadState = !state in   
if saveState false then 1 else loadState true
A visualisation of the execution is given below:

Spartan-like graph rewriting machines can go even far beyond, modelling more exotic operations such as found in data-flow like languages as can be seen in this separate on-line visualiser which shows the execution of typical examples such as finite-input-response filters.

To summarise what we have learned so far, the main ideas are:

  • Pure functional programming languages are algebras with resource management, achieved via thunking (time) and copying (space). The syntactic mechanism for managing copying are variables.
  • Impure languages are languages which are also capable of sharing. The syntactic mechanism for managing sharing are names.
  • The Spartan Universal machine can represent a very broad range of operations while keep track of costs accurately. 
In the next post I will talk about reasoning about properties of programs using the Spartan calculus. 

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