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It's not a black box in the sense you can move it around, evaluate later, not evaluate at all, etc. There is fundamentally nothing you can inspect about a gener
by pestaa 12y ago
It's not a black box in the sense you can move it around, evaluate later, not evaluate at all, etc. There is fundamentally nothing you can inspect about a general IO operation: it's a handle to a computation dealing with the external world.
- millstone 12y agoI'm pretty sure you can move black boxes around in the real world too :) There are definitely sensible things to do with "general IO operations." For example, consider `withArgs`, which replaces argv for the duration of an action. A natural approach is to pick through the received action, and replace all calls to `getArgs` with `return tempArgs`. But `withArgs` doesn't work this way because Haskell can't pick apart IO actions. Instead it uses a nasty FFI to modify a global. (It doesn't even look thread-safe: I'll bet withArgs "leaks" into other threads.) It's also overbroad: IO encapsulates "real world" computations like deleting files, but also basic operations like getting argv! I'd argue it's a failure of Haskell that the type system does not distinguish between a function that erases your hard drive and a function that gets your command line arguments.
- pestaa 12y agoYou either enforce 100% purity, or your language is not pure. Haskell chose to walk the former path. In that case reading global state needs to happen in the IO monad even if it is reading a couple of command line arguments. Though I completely agree it is nasty to change them in runtime, it is also assumed you know what you're doing when you tinker with them (cross-thread implications included.) To a Haskell program, command line arguments are exactly the same outer world as your super important files.