Dan Ingalls

Dan Ingalls is the implementer who made Smalltalk real. Where Alan Kay supplied the vision, Ingalls built the working systems — the bytecode engines, the BitBlt graphics primitive, and the live environment in which a programmer can change a running program and see the effect at once. His maxim, "a system should be built so you can see it work," captures the spirit of image-based, live development.

His paper Design Principles Behind Smalltalk remains a touchstone, and the lineage of his work runs through Squeak and on to Scratch. The idea that software is a living thing you converse with — not a text you compile and hope — owes a great deal to him. See the big idea of messaging and Alan Kay & Smalltalk.