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Things that make developing Psalm complicated

This is a somewhat informal list that might aid others.

Type inference

Type inference is one of the big things Psalm does. It tries to figure out what different PHP elements (function calls, if/for/foreach statements etc.) mean for the data in your code.

Within type inference there are a number of tricky areas:

Loops

Loops are hard to reason about - break and continue are a pain. This analysis mainly takes place in LoopAnalyzer

Combining types

There are lots of edge-cases when combining types together, given the many types Psalm supports. Type combining occurs in TypeCombiner.

Logical assertions

What effect do different PHP elements have on user-asserted logic in if conditionals, ternarys etc. This logic is spread between a number of different classes.

Generics & Templated code

Figuring out how templated code should work (@template tags) and how much it should work like it does in other languages (Hack, TypeScript etc.) is tricky. Psalm also supports things like nested templates (@template T1 of T2) which makes things trickier

Detecting dead code

Detecting unused variables requires some fun data-flow analysis.

Detecting unused classes and methods between different runs requires maintaining references to those classes in cache (see below).

Supporting the community

Supporting formal PHPDoc annotations
Supporting informal PHPDoc annotations e.g. ArrayIterator|string[] to denote an ArrayIterator over strings
non-Composer projects e.g. WordPress

Making Psalm fast

Parser-based reflection

Requires scanning everything necessary for analysis

Forking processes** (non-windows)

Mostly handled by code borrowed from Phan, but can introduce subtle issues, also requires to think about how to make work happen in processes

Caching thing

see below

Cache invalidation

Invalidating analysis results

Requires tracking what methods/properties are used in what other files, and invalidating those results when linked methods change

Partial parsing

Reparsing bits of files that have changed, which is hard

Language Server Support

Handling temporary file changes

When files change Psalm figures out what's changed within them to avoid re-analysing things unnecessarily

Dealing with malformed PHP code

When people write code, it's not always pretty as they write it. A language server needs to deal with that bad code somehow