+++
title = "`interface`"
weight = 1
+++

Defines an interface for a hardware module (see section
[5.13](fixme)). An
interface is essentially a `struct`, but its components are restricted
to those things that have a physical interpretation as wires in and out
of a circuit. The types of fields in an interface are more likely to
involve `Action`'s (see section
[5.11](fixme)),
which are typically interpreted as "enable signals" into a circuit. The
fields of an interface are also known as *methods* (not to be confused
with methods of a class, described in Sections
[2.1](fixme) and
[4.5](fixme)).

```
topDefn ::= interface typeId {tyVarId }= { { fieldDef ; }}
```

Example:

```hs
interface Stack a =
    push :: a -> Action
    pop :: Action
    top :: Maybe a
```

This describes a circuit that implements a stack (a LIFO) of items. This
polymorphic definition does not specify the type of the contents of the
stack, just that they have some type "`a`". Corresponding to the `push`
method, the circuit will have input wires to carry a value of type
"`a`", and a "push-enable" input wire that specifies when the value
present on the input wires should be pushed on the stack. Corresponding
to the `pop` component, the circuit will have a "pop-enable" input wire
that specifies when a value should be popped off the stack.
Corresponding to the `top` component, the circuit will have a set of
output wires: if the stack is empty, the wires will represent the value
`Nothing`, and if the stack is non-empty and $v$ is the value at the top
of the stack, the wires will represent `Maybe` $v$.