2.7.8.1 Field

The virtual machine supports a generalization of the left and right deconstruction operations that is applicable to deeply nested structures. Use of this feature is conducive to code that is faster and more compact than is possible by relying on the primitive deconstructors alone. It may also be easier for a code optimizer to recognize and transform.

The general form of a virtual code application to perform deconstruction is that it is a pair with a nil left side, and a non-nil right side. The right side indicates the nature of the deconstruction to be performed when the function is evaluated on an argument.

To make the expression of deconstruction functions more readable in silly, the standard library contains the declaration

field = couple(constant nil,identity)

which implies the following theorem.

T13

[[field]] w = (nil,w)

The virtual machine recognizes an application in this form and evaluates it according to the following properties, where u and v are other than nil, but x, y, and z are unrestricted.

P10

([[field]] (u,nil)) (x,y) = ([[field]] u) x

P11

([[field]] (nil,v)) (x,y) = ([[field]] v) y

P12

([[field]] (u,v)) z = (([[field]] u) z,([[field]] v) z)

One might also add that ([[field]] (nil,nil)) z = z, but this statement would be equivalent to P0.

A suitable choice of the field operand permits the implementation of any deconstruction function expressible in terms of compose, couple, identity, left and right. For example, the application couple(compose(right,right),left) has an equivalent representation in field((nil,(nil,(nil,nil))),((nil,nil),nil)). The latter looks longer in silly but is smaller and faster in virtual code.

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