The Sifflet visual functional programming language and aid to understanding recursive functions (

#46Errors involving a node with insufficient inputs.

Given this incorrect definition of the power function:

Power x n = if (zero? n)
            (* x (Power (sub1 n)))

-- note that in the recursive step, the call to Power has only one argument -- and giving inputs x = 3, n = 6, trying to expand the Power node (the recursive call) causes the program to crash.

O: 2012 Nov 12 R: A Starks P: high

    • added tag p:high
  • Saving the function definition and evaluating the function call are okay (Sifflet displays stack overflow errors on the diagram, which is right because the second argument to Power never changes). On trying to expand the recursive Power node, sifflet crashes with this error message:

    sifflet-devel: buildFooterText: mismatched lists

  • 2013 July 4.

    Sifflet infers the type of this function as (Num -> (Num -> Num)), which seems wrong because of the missing argument to the recursive Power node.

    Indeed, ghci reports an "occurs check" error for the following definition:

    power x n = 
      if n == 0
      then 0
      else x * power (n - 1)
        Occurs check: cannot construct the infinite type: a0 = a0 -> a0
        In the return type of a call of `power'
        Probable cause: `power' is applied to too few arguments
        In the second argument of `(*)', namely `power (n - 1)'
        In the expression: x * power (n - 1)
    • added tag p:medium
    • removed tag p:high

    Partly fixed. Sifflet no longer crashes in this case, but displays the arguments without corresponding inputs as, e.g., "x = ?".

    This is no longer high priority, but there are a couple of concerns:

    1. The type checker should reject function definitions of this kind.

    2. Even if it does not, the evaluator should produce a different kind of error than stack overflow, since the value of partially applied Power is not a number, but a function of type (Num -> Num), and therefore unsuitable as input to the '*' function.

    • summary changed to "Errors involving a node with insufficient inputs."