`type`

Defines a type synonym. These are used purely for readability, i.e., a type synonym can always be "expanded out" to its definition at any time.

topDefn ::= type typeId {tyVarId }= type

Examples:

type Byte     = Bit 8
type Word     = Bit 16
type LongWord = Bit 32

These provide commonly used names for certain bit lengths. In a specification of a processor:

data RegName = R0 | R1 | ... | R31
type Rdest = RegName
type Rsrc = RegName
data ArithInstr = Add Rdest Rsrc Rsrc
                | Sub Rdest Rsrc Rsrc

the last two lines suggest the roles of the registers in the instructions, and is more readable than:

data ArithInstr = Add RegName RegName RegName
                | Sub RegName RegName RegName