 Modula-2

Modula-2 is a systems programming language designed by Niklaus Wirth, combining structured programming with explicit software modules and facilities for low-level system construction. A definition module declares an exported interface; an implementation module supplies its implementation and private details. Clients import the declared names instead of depending on every declaration in the implementation.

For example, a stack module can export a stack type and push/pop procedures while keeping the array or linked representation private. This permits separate compilation against an interface and replacement of the implementation when its contract is preserved. Wirth's project account places Modula-2 in the Lilith workstation effort and records completion of its first compiler at ETH in1979.^[ref] Low-level facilities make hardware-oriented programs expressible, but also require care about the assumptions connecting language operations to devices and machine representations.