Defunctionalization (d17n) is a whole program transformation that turns higher-order functional programs into first-order ones. It's simplest to divide it into four or five stages, though it is really a single transformation.
A simple example
Let's use as an example a completely pointless program that uses map twice:
prog x ys = (map aug ys, map sqr ys) where aug y = x + y sqr y = y * y
Because d17n works on whole programs, we'll have to include the defintion of map:
map :: (Integer -> Integer) -> [Integer] -> [Integer] map f  =  map f (x:xs) = f x : map f xs
(Note that, to avoid complications, I've given map a type that contains no type variables, despite the fact that its definition also supports the more general type (α → β) → [α] → [β].)
The first step is to decode the dataflow, that is, to isolate the expressions that create or use or pass on the value of higher-order arguments or results. The simplest way of doing this is to introduce a data type that encapsulates these values, together with a trivial function apply that unwraps them when necessary. In our program, we want to get rid of the functional argument f in map f xs. Let's isolate that argument by introducing a data type Func, then rewrite prog so that it wraps up each functional argument as a Func and rewrite map so that it uses a new function apply to unwrap the functional argument:
data Func = Func (Integer -> Integer) prog x ys = (map (Func aug) ys, map (Func sqr) ys) where aug y = x + y sqr y = y * y map :: Func -> [Integer] -> [Integer] map f  =  map f (x:xs) = apply f x : map f xs apply :: Func -> Integer -> Integer apply (Func f) x = f x
So far apply is trivial; but later it will collect all the code for functions that are passed to map.
The second step is to look for lambdas, that is, to identify the functions that get wrapped up in the new data type and passed as functional arguments. I say 'look for the lambdas' because often these functions are written as lambda-expressions, though as our example shows, this need not be the case. Let's re-express our program using lambdas:
prog x ys = (map (Func (\ y -> x + y) ys, map (Func (\ y -> y * y) ys))
The point of doing this is so that we can find the free variables of each lambda-expression. The function (λ y → y * y) is always the same: it returns the square of its argument. But the function (λ y → x + y) depends on x, and we need to remember the value of x in order to work out what the function returns.
Now we will create the constructors: that is, redefine the data type Func to reflect the lamda-expressions we have found. We are going to represent the function (λ y → y * y) by a constant Sqr, but we are going to represent (λ y → x + y) by the constructed value Aug x that contains the value of x. Here is the appropriate data type:
data Func = Sqr | Aug Integer
We can rewrite the function prog to use these constructors in place of the lambda-expressions:
prog x ys = (map (Aug x) ys, map Sqr ys)
The final step is to write the worker: that is, to redefine apply because its task is no longer trivial. It now becomes a kind of interpreter for the tiny language that the Func data type has become. The new apply is defined by a set of equations, each associating one of the constructors we just introduced with the body of the lambda-expression it represents. Because the arguments of the constructor (such as Aug x) are exactly the free variables of the lambda-expression, everything matches up perfectly. In our example:
apply :: Func -> Integer -> Integer apply (Aug x) y = x + y apply Sqr y = y * y
It's important to see that d17n must be applied to a whole program at once, since creating the data type Func and the function apply require us to identify all the places where calls to map occur so as to capture the lambda-expressions from those call sites. Thus d17n compromises the modularity of programs, because it means that the resulting program cannot any longer be decomposed into independent pieces.
Beginning with the interpreter for pure Fun, we want to eliminate some of the uses of higher-order functions. Specifically, let's try to get rid of the alternative in the type Value that reads
data Value = ... | Function ([Value] -> Value) | ...
We'll use a program transformation called defunctionalization or d17n for short. This is a whole-program transformation that replaces higher-order functions by first-order data structures, but also in the process makes the program less modular and more monolithic. There are about four stages:
Decoding the dataflow
In our program, this is easy, because the values we are interested in are already wrapped in the Function constructor. These values are created in two places – the function abstract that says,
abstract xs e env = Function h where h args = eval e (defargs env xs args),
and in the function mkprim that produces a pair consisting of a name and a primitive function,
mkprim x = (x, Function (prim x)).
Function values are used in one place – the function apply, defined by
apply (Function h) args = h args.
(In general, we might need to analyse the program a bit more carefully to isolate a set of places where higher-order values are created and used. Wrapping the values in a data-type is a good way to make this information explicit.)
Looking for lambdas
Now we'll get rid of auxiliary functions and partial applications by writing each creation site as a lambda expression. So we write
abstract xs e env = Function (\ args -> eval e (defargs env xs args))
mkprim x = (x, Function (\ args -> prim x args)).
The point of introducing lambdas is that it makes absolutely clear what are the free variables. In abstract, they are xs, e and env; and in mkprim the only free variable is x. Other names like defargs, eval and prim are globally defined, so there is no need to capture them as part of the constructed value.
Creating the constructors
For each creation site, we want a constructor for values that packages the free variables of the lambda expression. So we will delete the alternative Function from the Value data-type, and replace it with
data Value = ... | Closure [Ident] Expr Env | Primitive String | ...
Then we redefine abstract so that it packages the free variables xs, e and env as a Closure:
abstract xs e env = Closure xs e env
(so abstract has become trivial). And we redefine mkprim in a similar way:
mkprim x = (x, Primitive x).
Writing the worker
The lambda bodies that we deleted from the creation sites are collected together in a new version of apply, which uses the saved ingredients to carry out the same computation as before:
apply (Closure xs e env) args = eval e (defargs env xs args) apply (Primitive x) args = prim x args.
The transformation is now complete.
We have succeeded in replacing the higher-order value type with a first-order type that's much less frightening. And we've also had a good effect on the equation that defines recursion:
elab (Rec x (Lambda xs e)) env = env' where env' = define env x (Closure xs e env')
This can now be read as describing a data structure with a cycle in it, rather than some weird kind of recursive function.
Transforming away higher-order functions like this gives us a double boost of confidence in using them in writing definitional interpreters. First, we know that if we want to avoid higher-order functions for foundational reasons, we can transform them away. And second, if we want to relate our abstract interpreters to implementations, d17n gives us a way of introducing concrete data structures.