Wall

# Functions III

Some functions in Wall mimic control structures in imperative languages. For example, if statements and loops are accomplished with functions in Wall. Below are some common control functions.

# Question mark

The question mark in Wall is a function that accepts true or false, an value in case of true and value in case of false.

w> ? false 5 6
6

A related function, ??, accepts true or false, a function to evaluate in case of true and a function to evaluate in case of false. The function is evaluated with arbitrary input.

w> ?? false (always 4) (always 5)
5

# flip

Flip takes three arguments: a function and two arguments (call them a and b to that function). It then invokes the function with b followed by a instead of a and then b. For example, we can define a mirror function to >= called >=' using flip.

w> >=' = flip <
w> >=' 4 3
true
w> >=' 4 4
true
w> >=' 4 5
false
w> == >=' >=
true

# filt

filt filters a list or funtion based on a predicate applied to the values:

w> filt [1 2 3 4] (< 2)
[3 4]
w> filt {55: 1, 63: 2, 77: 3, 89: 4} (< 2)
{77: 3, 89: 4}

A version for sets, filt-s, applies a predicate to values in a set.

w> filt-s :[1 2 3 4] (< 2)
:[3 4]

# map

map maps values from a list, set or function to another list, set or function.

w> map [1 2 3] (+ 3)
[4 5 6]
w> [1 2 3].map (+ 3)
[4 5 6]
w> [1 2 3].map (* 0)
[0 0 0]
w> :[1 2 3].map (* 0)
[0]
w> {1: 2, 3: 4, 5: 6}.map (* 0)
{ 1: 0, 3: 0, 5: 0 }

A version for sets, map-s, applies a mapping function to keys in a set that map to true.

w> map-s :[1 2 3 4] (+ 1)
:[2 3 4 5]

# red

red is used to perform a reduction over a function:

  • the first argument is the function;
  • the second argument is a sorting function for values in he first argument;
  • the third argument is a function that acts on the aggregator and the next value; and
  • the final argument is an initial value to serve as the aggregator.
w> red [1 2 3] < + 0
6

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