Notes from Thomas Greco

Software engineer. TypeScript, functional programming, and the things I build on weekends. Some posts first appear at Hindley Milner Systems and are mirrored here; the rest are mine alone.

$Hand-inked isometric drawing of an 80 mm refractor telescope on a tripod, with a flat blue cast shadow$

Fig. 21 — Telescope, 80 mm f/10 on an EQ2 mount

Key written here mirrored from hindleymilner.com

27 Oct 2026λ hindleymilner.com4 min
Prevent Impossible States Pt. 1 - Union Types
Use TypeScript discriminated unions to require the right fields, narrow object types, and catch unhandled cases.
typescript union-types category-theory series
1 May 2018tgrecojs.com5 min
Fun with Functors
JavaScript arrays are functors. What that means, the two laws they follow, and how to build a Box of your own.
javascript functional-programming functors

Prevent Impossible States Pt. 1 - Union Types

Fun with Functors