Truthiness in code

25 Oct 2020 @206

Programming languages have different ideas about what is true and what is false.

Truthy languages

In Forth, any non-zero values are treated as true.

Code example

1 1 =  \ true, pushes -1 onto the stack
1 2 =  \ false, pushes 0 onto the stack

: TRUE? ( n -- )
    IF ." yes" ELSE ." no" THEN
;
1  TRUE?  \ prints "yes"
-1 TRUE?  \ prints "yes"
0  TRUE?  \ prints "no"

In Racket, any value other than false (#f) is treated as true.

Code example

(if #f "true" "false")  ; "false"
(if #t "true" "false")  ; "true"
(if 0  "true" "false")  ; "true"

In Ruby, any value except false and nil is treated as true.

Code example

def assert!
  raise unless yield
end

def falsy_values
  assert! { not false }
  assert! { not nil }
end

def truthy_values
  assert! { true }
  assert! { 0 }
  assert! { '' }
  assert! { [] }
  assert! { {} }
end

falsy_values
truthy_values

Falsy languages

In C, zero values, null values, and null pointers are falsy. Pointers to falsy values are treated as true (e.g. empty strings).

Code example

#include <assert.h>
#include <stdlib.h>

void falsy_values()
{
    const int   i = 0;      assert(!i);
    const float f = 0.0;    assert(!f);
    const char  c = '\0';   assert(!c);
    const char *s = NULL;   assert(!s);
}

void truthy_values()
{
    const int   i = 1;      assert(i);
    const float f = 1.0;    assert(f);
    const char  c = '0';    assert(c);
    const char *s = "";     assert(s);
}

int main()
{
  falsy_values();
  truthy_values();
  return 0;
}

In JavaScript, zero values, null values, and empty strings are falsy. However, empty arrays and empty objects are treated as true.

Code example

function falsy_values() {
  if (false)     { throw '!' }
  if (null)      { throw '!' }
  if (undefined) { throw '!' }
  if (NaN)       { throw '!' }
  if (0)         { throw '!' }
  if ('')        { throw '!' }
}

function truthy_values() {
  if (!true)     { throw '!' }
  if (!1)        { throw '!' }
  if (!'0')      { throw '!' }
  if (![])       { throw '!' }
  if (!{})       { throw '!' }
}

falsy_values()
truthy_values()

In Python, blank/empty values are generally falsy: zero values, empty strings, empty lists, and so on.

Code example

def falsy_values():
    assert not False
    assert not None
    assert not 0
    assert not ''
    assert not ()
    assert not []
    assert not set()
    assert not {}

def truthy_values():
    assert True
    assert 1
    assert '0'
    assert (None,)
    assert [None]
    assert {None}
    assert {None: None}

falsy_values()
truthy_values()