I

I is an experimental language by Marshall Lochbaum, based on the concept of extending the implicit mapping used by array languages to all datatypes including functions. Lochbaum has described the language as "not usable" and stated that he tried to avoid the constraining nature of I's design decisions in his later language BQN.

Like K, the language uses nested lists rather than arrays in order to simplify the design. Although a related paper defines mapping over associative arrays, references, and sets, as of 2021 the I implementation only supports lists and functions as datatypes. I uses a left-to-right syntax that allows whitespace to indicate precedence in addition to parentheses. Every value can be called as a function, and every function takes two arguments, with the function  used to call functions of one argument. Primitive functions are written with single ASCII characters, while identifiers are surrounded with single quotes. I has no function definition syntax; instead functions must be defined using tacit programming. As a result there is only a single global scope for variables.

I is implemented using soft typing and JIT compilation for many operations, targetting x86-64 directly.