# Chapter 30. Strict functions

As an experimental mode called strictFuncs a stricter definition of “side
effect” is available. In addition to the existing rule that a side effect is calling
a function with side effects the following rule is also enforced:
A store to the heap via a ref or ptr indirection is not allowed.
For example:
 {.experimental: "strictFuncs".}
 type
    Node = ref object
      le, ri: Node
      data: string
 func len(n: Node): int =
    # valid: len does not have side effects
    var it = n
    while it != nil:
      inc result
      it = it.ri
 func mut(n: Node) =
    n.data = "yeah" # short for: m[].data = "yeah"
    # Error: 'mut' can have side effects
Mutations via var T and out T parameters continue to be allowed in a strict
func. Hence a prototype such as func add[T](s: var seq[T]; elem: sink T) is
valid and can be used in the following func:
 func tokenize(input: string): seq[string] =
    result = @[]
    for w in splitWhitespace(input): result.add w
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The mutation that add performs does not count as a side effect and so
tokenize can be a func.