Language Design: Unified Condition Expressions

Introduction

Idea

Replace the different syntactic forms of

• if expressions,
• pattern matching and pattern guards,
• if-let constructs

with a single, unified condition expression that scales from simple one-liners to complex pattern matches.

Motivation

The intention is to cut the different syntax options down to a single one that is still easily recognizable by users, not to minimize keywords (i. e. a == b ? c : d) or turn conditions into methods (like Smalltalk).

Principles

• The condition can be split between a common discriminator and individual cases.
  • This requires doing away with mandatory parentheses around the conditions.
  • This strongly suggests using a keyword (then) to introduce branches, instead of using curly braces, based on readability considerations.
• The keyword if is chosen over other options like match, when, switch or case because it is keyword the largest number of developers are familiar with.

Examples

The following examples assume that the language has indentation-sensitive syntax to ensure unambiguous parsing.

Languages without indentation-sensitive syntax require either mandatory braces around the bodies of then branches, or ending then branches explicitly, for instance with end or a ,.

simple if expression

if x == 1.0                         /* same as */
then "a"                            if x == 1.0 then "a" else "z"
else "z"

one comparison operator on multiple targets

if x ==                 if x                    /* same as */
  1.0 then "a"            == 1.0 then "a"       if x == 1.0      then "a"
  2.0 then "b"            == 2.0 then "b"       else if x == 2.0 then "b"
      else "z"                   else "z"       else                  "z"

different comparison operators, equality and identity

if x                                /* same as */
  ==  1.0 then "a"                  if x == 1.0       then "a"
  === NaN then "n"                  else if x === NaN then "b"
          else "z"                  else                   "z"

method calls

if xs                               /* same as */
  .isEmpty       then "e"           if xs.isEmpty            then "e"
  .contains(0.0) then "n"           else if xs.contains(0.0) then "n"      
                 else "z"           else                          "z"

pattern matching (is), introducing bindings ($)

if alice
  .age < 18                  then "18"
  is Person("Alice", $age)   then "$age"
  is Person("Bob", _)$person then "{$person.age}"
                             else "0"

pattern matching using “if-let”¹²

if person is Person("Alice", $age) then "$age" else "o"

wildcards (_) and pattern guards

if person                         /* same as */      if person is
  is Person("Alice", _)           then "alice"         Person("Alice", _)           then "alice"
  is Person(_, $age) && age >= 18 then "adult"         Person(_, $age) && age >= 18 then "adult"
                                  else "minor"                                      else "minor"

Related Work

• Haskell – multi-way if-expressions
• CommonLisp – cond and case