Functional Dependencies

関連論文

引数同士の関数関係を明示する

これを

code:purs(hs)

class Stream stream element where

uncons :: stream -> Maybe { head :: element, tail :: stream }

こう書く

code:purs(hs)

class Stream stream element | stream -> element where

uncons :: stream -> Maybe { head :: element, tail :: stream }

例

code:purs(hs)

class Stream stream element | stream -> element where

uncons :: stream -> Maybe { head :: element, tail :: stream }

instance streamArray :: Stream (Array a) a where

uncons = Array.uncons

instance streamString :: Stream String String.CodePoint where

uncons = String.uncons

instance streamString :: Stream String Int where -- error!!

uncons xs = Just { head: 1, tail: xs }

instance streamString :: Stream Int String.CodePoint where -- error!!

uncons xs = Just { head: String.codePointFromChar 'c', tail: xs }

下に書いたほうがエラーになる

コドメインが被っていてもエラーになる

例

code:purs(hs)

class Union (left :: # Type) (right :: # Type) (union :: # Type)

| left right -> union, right union -> left, union left -> right

3つの型引数を取っている

その中の任意の2つの型が決まれば、残りの一つも決まる、ということを言っっている

例

code:purs(hs)

class Cons (label :: Symbol) (a :: k) (tail :: Row k) (row :: Row k)

| label a tail -> row, label row -> a tail

4つの型引数を取っている