One-liners

In the case of a well-known conversational programming language I have been told from various sides that as soon as a programming community is equipped with a terminal for it, a specific phenomenon occurs that even has a wellestablished name: it is called “the one-liners”. It takes one of two different forms: one programmer places a one-line program on the desk of another and either he proudly tells what it does and adds the qustion, “Can you code this in less symbols?” — as if this were of any conceptual relevance! — or he just says, “Guess what it does!” From this observation, we must conclude that this language as a tool is an open invitation for clever tricks; and while exactly this may be the explanation for some of is appeal, viz. to those who like to show how clever they are, I am sorry, but I must regard this as one of the most damning things that can be said about a programming language.

E. W. Dijkstra (1987): The Humble Programmer.

Table of contents

1. Map, filter, reduce
2. Chaining lists
3. Quicksort
4. Permutations
5. Number theory
6. Currying
7. Partial application
8. Set theory
9. Mandelbrot set
10. Fizz buzz

Map, filter, reduce

List.map = |a;f| [] if a==[] else [f(a[0])]+a[1..].map(f)

List.filter = |a;p| ([] if a==[] else
   ([a[0]] if p(a[0]) else [])+a[1..].filter(p))

List.reduce = |a;e,f| e if a==[] else f(a[0],a[1..].reduce(e,f))


List.map = |a;f| a.reduce([],|t,x| t+[f(x)])

List.filter = |a;p| a.reduce([],|t,x| t+[x] if p(x) else t)

Chaining lists

chain = |a| list(x for t in a for x in t)

a = list((1..10).chunks(2))
print(chain(a))

Quicksort

use math.partition

qsort = |a| [] if a==[] else qsort[a[1..]%|x| x<a[0]].chain(a[0])

Permutations

# Print all permutations of [0,1,2].
(list(3)^3).filter(|t| (0..2).all(|x| x in t))).each(print)

Number theory

# Print the first 100 prime numbers.
print((1..).filter(|n| (1..n).count(|k| n%k==0)==2).list(100))


# Calculate the prime factorization of 360.
isprime = |n| (1..n).count(|k| n%k==0)==2

factor = |n| (list(2..n).filter(|k| n%k==0 and isprime(k))
   .map(|p| [p,(1..).until(|k| n%(p^k)!=0).count()]))

print(factor(360))

Currying

# Convert f(x,y) to f(x)(y).
curry = fn|f| n = f.argc(); a = list(n);
   (2..n).reduce(fn|x| a[n-1] = x; f(*a) end,
   |g,i| fn|x| a[n-i] = x; g end) end


# Convert f(x)(y) to f(x,y).
uncurry = |f| |*a| a.reduce(f,|g,x| g(x))

Partial application

# Variadic, fixes first arguments.
first = |f,*a| |*b| f(*(a+b))

# Variadic, fixes last arguments.
last = |f,*a| |*b| f(*(b+a))

Set theory

# Calculate the power set of {0,1,2}.
P = |a| a.reduce({{}},|p,x| p|set(p.map(|s| s|{x})))
print(P({0,1,2}))

Mandelbrot set

(0..25).map(|x|
   (0..39).map(fn|y| c = (x/10-2)+(y/10-2)*1i;
      "*" if abs(((|z| z^2+c)^10)(0))<2 else "\s"
   end).join()).each(print)

Fizz buzz

(1..100).map(|i| [i]+[3,5].map(|d| int(i%d==0))).map(|[i,x,y]|
   i if x+y==0 else "Fizz"*x+"Buzz"*y).each(print)