Type System

Egison has static type system from version 4.

Type Declarations

Types are declarared as follow.

(define $map : (($A / $B) {A} / {B})
  (lambda [$fn $xs]
    (match xs (list something)
      {[<nil> {}]
       [<cons $x $rs> {(fn x) @(map fn rs)}]})))

Type Classes

We can use type classes as follow.
We don't support multi-parameter type classes.

(type $Ordering {<Less> <Equal> <Greater>})

(define $ordering : (Matcher Ordering)
  (algebraic-data-matcher
    {<less> <equal> <greater>}))

(class $Ord $A :: Eq
  {[$lt? : (A A / Bool)]}
  {[$lte? : (A A / Bool) (lambda [$x $y] (or (lt? x y) (eq? x y)))]
   [$gte? : (A A / Bool) (lt? $2 $1)]
   [$gt? : (A A / Bool) (lte? $2 $1)]
   [$compare : (A A / Ordering)
    (lambda [$m $n]
      (if (lt? m n)
        <Less>
        (if (eq? m n)
          <Equal>
          <Greater>)))]})

(instance Integer Ord {[$lt? lt-integer?]})

Static type system #48

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