Language Design: Unified Condition Expressions
Introduction
Published on 2018-01-21.
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 likematch
,when
,switch
orcase
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