问题描述
有时,如果模式规则需要一些特殊的rhs,通过where
使其更具可读性,我最终会得到这样的东西
data D = A | B | C
func :: D -> b
func A = special_case
where
special_case = other helper
other = aaaa
helper = bbb
func _ = bla
由于 where
很长,所以包罗万象的模式似乎与其他模式相去甚远。如果我能写出这样的东西就好了:
func :: D -> b
func !A = bla -- imaginary Syntax
func A = special_case
where
special_case = other helper
other = aaaa
helper = bbb
我不认为它仍然会被称为“全能”,而是“全能但”,但有什么办法可以做到这一点吗?
解决方法
如果你不需要绑定任何东西,你可以这样做:
isA :: D -> Bool
isA A = True
isA _ = False
func :: D -> b
func d | not (isA d) = bla
func A = special_case where ...
(你也可以实现 isn'tA
来避免 not
。但是虽然我经常看到像 isA
这样定义的函数,但我相信我从来没有见过isn'tA
的类比。)
对于编译器来说,这个匹配是否完成可能并不明显。你可以这样解决:
type A'sFields = () -- your specific D doesn't have any fields for A,-- but this pattern is general enough to use for
-- constructors that do
fromA :: D -> Maybe A'sFields
fromA A = Just ()
fromA _ = Nothing
func :: D -> b
func d = case fromA d of
Nothing -> bla
Just () -> special_case where ...