Why can't I call a non-pure function from a pure function?

← Purity Contracts · Ref: Q791

A pure function can only call other pure functions and methods. Calling a non-pure function would allow side effects to leak into the pure context, breaking the purity guarantee.

THE PURITY CHAIN

If A is pure and calls B, then B must also be pure. If B calls C, then C must be pure too. The entire call chain from a pure function must be pure.

THIS EXAMPLE

The double() and quadruple() functions are both pure. quadruple() calls double() — valid because double() is also pure. The logResult() function is NOT pure (it uses Stdout). The mutation adds a call to logResult() inside quadruple().

See Q43 for pure function basics. See Q790 for reassignment in pure.

Example

defines module qa.puritycontracts.impurecall

  defines function

    double() as pure
      -> input as Integer
      <- rtn as Integer: input * 2

    quadruple() as pure
      -> input as Integer
      <- rtn as Integer?

      rtn: double(double(input))

    logResult()
      -> result as Integer
      stdout <- Stdout()
      stdout.println($result)

  defines program

    ShowPurity()
      stdout <- Stdout()
      result <- quadruple(5)
      logResult(result)
      stdout.println($result)

Common mistakes

E08130 — Calling the non-pure function logResult() from a pure function is not allowed. Pure functions can only call other pure functions. See ek9 -h E08130 for details.

Incorrect:

      rtn: double(double(input))
      logResult(rtn)

Correct:

      rtn: double(double(input))

Coming from another language?

Java: no purity enforcement. Python: no purity concept. Rust: no function-level purity, uses ownership for safety. Kotlin: no purity concept. Haskell: IO monad separates pure from impure. EK9: explicit 'as pure' with transitive enforcement — the entire call chain must be pure.

Keywords: function, E08130, side effect, call, chain, pure, impure