Function Evaluation in Julia and R: Example 1.5 from SICP

01-bitdiddle.jl

# Ben Bitdiddle's demonstration that Julia uses applicative-order evaluation

p() = p()

# p (generic function with 1 method)

test(x, y) = x == 0 ? 0 : y

# test (generic function with 1 method)

test(0, p())

# ERROR: stack overflow
#  in p at none:1 (repeats 80000 times)

02-bitdiddle.R

# In contrast, R uses a form of delayed evaluation that is more similar
# to normal-order evaluation

p <- function() {p()}

test <- function(x, y) {
    if (x == 0L) {
        return(0L)
    } else {
        return(y)
    }
}

test(0L, p())

# [1] 0

03-bitdiddle.py

# Like Julia, Python also uses applicative-order evaluation

def p():
    p()

def test(x, y):
    if x == 0:
        return 0
    else:
        return y

test(0, p())

# Traceback (most recent call last):
#   File "<stdin>", line 1, in <module>
#   File "<stdin>", line 2, in p
#   [...]
#   File "<stdin>", line 2, in p
# RuntimeError: maximum recursion depth exceeded

04-bitdiddle.rb

# Like Julia and Python, Ruby also uses applicative-order evaluation

def p()
    p()
end

def test(x, y)
    if x == 0
        return 0
    else
        return y
    end
end

test(0, p())

# SystemStackError: stack level too deep
#   from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/irb/workspace.rb:86
# Maybe IRB bug!