1.	Introduction
2.	Elaboration and Compilation
3.	The Type System
4.	Source Files
5.	Recursive Definitions
6.	Terms
7.	Type Classes
8.	Functors, Monads and `do`-Notation
9.	IO
10.	Tactic Proofs
11.	The Simplifier
12.	Basic Types
13.	Standard Library
14.	Notations and Macros
15.	Output from Lean
16.	Elan
17.	Build Tools and Distribution
	Index

9. IO

9.1.	Logical Model
9.2.	Control Structures
9.3.	Console Output
9.4.	Mutable References
9.5.	Files, File Handles, and Streams
9.6.	Environment Variables
9.7.	Timing
9.8.	Processes
9.9.	Random Numbers
9.10.	Tasks and Threads

9.6. Environment Variables

9.6. Environment Variables🔗

🔗opaque

IO.getEnv (var : String)
    : BaseIO (Option String)