Not only some people program whole games in fragments shaders (like Quakes or others), but also some simulate whole computers or programming language interpreters !
For now there are 8 of them:
Apple I emulator ( ROM + memory editor + BASIC language ). More about Apple_I computer.
Turing Machine Interpreter . More about Turing Machine.
Chip-8 Emulator ( ROM + compiles ). More about Chip-8 language.
Simple bytecode interpreter ( assembly language ). More about bytecode.
Core war ( red code for game fight ). More about Core war.
Shader Redstone 3 ( in-minecraft code ) More about Redstone.
Shader Redstone 2.
Commodore 64 (toy Basic interpreter with just PRINT, GOTO, LIST, RUN).
‘Little Man Computer’ Simulator (Assembly language – educational model of a computer)
See all emulators in Shadertoy.
See all interpreters in Shadertoy.
shadertoy - unofficial 
C64 https://www.shadertoy.com/view/Xs3XW4
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yes, thanks !
( ok it’s a toy interpreter, but still 🙂 )
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