IMPURE 

"Modifier: Non-deterministic or consults non-FINAL external state"
return: [action! frame!]
^value [action! frame!]
:off "Give back non-pure version of the passed in function"

IMPURE marks a function as one that PURE code is not allowed to call. Use it on a function whose result can vary even with the same arguments, or which has effects beyond its arguments: talking to the user, the filesystem or the network, reading the clock, or generating random numbers.

/coin-flip: impure does [random-pick [heads tails]]
assert [find [heads tails] coin-flip]

Calling an IMPURE function from pure code is an error, even through a FINAL variable and even indirectly:

best-of-three: pure does [reduce [coin-flip coin-flip coin-flip]]
best-of-three

Most Functions Are Neither

Functions start out neither PURE nor IMPURE. That’s on purpose: it lets things like IF and APPEND work in both pure and impure code. A function that isn’t marked only causes an error if it actually does something impure while it’s running under a pure function.

So why mark anything IMPURE, when the system would notice it reading a non-FINAL variable anyway? Because for natives, and for functions that get their non-determinism from outside the interpreter, there’s no variable access to notice. RANDOM doesn’t read a variable to pick its number.

Impure Natives

The natives marked IMPURE include:

  • Randomness: RANDOM, RANDOM-BETWEEN, RANDOM-PICK, RANDOMIZE, SHUFFLE, SHUFFLE-OF
  • Time: NOW
  • Output: WRITE-STDOUT (and so PRINT, which uses it)
  • Ports and files: CREATE, DELETE, OPEN, CLOSE, READ, WRITE, QUERY, MODIFY, RENAME
  • Undoing protections: MUTABLE, UNPROTECT

Some mezzanine functions written in Revolt are marked IMPURE as well, like ASK, CONFIRM, EXISTS?, and MAKE-DIR.

IMPURE:OFF

impure:off gives back a function that isn’t marked impure. A function can’t be both PURE and IMPURE, so to make an impure function pure you have to take the IMPURE mark off first.

IMPURE currently changes the function it’s given, rather than making a copy. Other references to the same function become impure too.

See Also