相关疑难解决方法(0)

类Applicative声明为：

class Functor f   =>  Applicative f   where
pure  ::  a   ->  f   a
(<*>) ::  f   (a  ->  b)  ->  f   a   ->  f   b

我们可以表示fmapi, i=0,1,2,...在条款pure和(<*>)：

fmap0 ::  a   ->  f   a
fmap0 =   pure
fmap1 ::  (a  ->  b)  ->  f   a   ->  f   b
fmap1 g   x   =   pure    g   <*> x
fmap2 ::  (a  ->  b   ->  c)  ->  f   a   ->  f   b   ->  f   c
fmap2 g …