Language Implementation Patterns

Pattern 21

Automatic Type Promotion

Purpose

This pattern describes how to automatically and safely promote arithmetic operand types.

Discussion

The goal of automatic type promotion is to get all operands of a single operation to be of the same type or compatible types. That’s just how computer CPU instructions want their operands. Unfortunately, that’s fairly inconvenient from a programming point of view. We often want to use a variety of types within the same operation such as 3+’0’. Most programming languages would, for example, interpret 3+’0’ as 3+(int)’0’.

A language application can convert between types at will as long as it doesn’t lose information. For example, we can convert 4 to 4.0 but can’t convert 4.5 to integer 4 without losing information. ...