# Modelling composite types

A composite type is one that can have different possible types of value, for example contact info might be either a phone number or an email address. Other common names for this—depending on programming language and context—are sum types, tagged unions, disjoint unions, discriminated unions, coproducts, or variant types (not the same as the old COM concept).

# The engineering career triangle

Engineering career progression is often described as a ‘ladder’ or ‘track’, where you can also switch into a parallel management track. However, this doesn’t adequately illustrate where the tech lead role sits, or why the staff+ engineer levels have at least as much in common with management as they do with coding. To be able to explain this we need to separate the concepts of management and leadership. The result is the engineering career triangle.

Identifiers aren’t usually considered very interesting. Indeed, popular frameworks such as Django or Hibernate find them so uninteresting that they ship with the default of auto-incrementing integers so users don’t need to be concerned with them. Unfortunately using sequential identifiers is a bad idea, and if you don’t think about your identifiers up front then fixing things later can be very time consuming. Here’s most of what you need to know about identifiers.

# Deliberate engineering

When building a software project with a given scope there are three main constraints you can trade off: good, fast, and cheap. This is commonly known as the project management triangle. Focusing on one will have an effect on the others. For example, trying to do things cheap will adversely affect good and fast.

# Tail recursion, fold, and more

People new to functional languages often struggle a bit with the concept of recursion, and in particular tail recursion. In this post we’ll look at both recursive and tail recursive functions using the substitution model to show their effect on the stack. We’ll also walk through deriving fold, map, and other functions from first principles because it fairly naturally follows on from this.