你会递归在列表的其余部分(你需要3起):
fun findItem (name, ((x,_)::firstlist)::tail) = x = name orelse findItem(name, firstlist::tail)
| findItem (name, []::tail) = findItem (name, tail)
| findItem(name, []) = false
但它更容易对眼睛,如果你先写,搜索通过列表,然后函数使用,在其他功能:
fun findItemHelper (_, []) = false
| findItemHelper (name, (n', _)::ns) = name = n' orelse findItemHelper (name, ns)
fun findItem (_, []) = false
| findItem (name, n::ns) = findItemHelper (name, n) orelse findItem (name, ns)
这些是除了orelse
之前的部分完全一样的,所以我们可以抽象说出来与谓词函数:
fun find (_, []) = false
| find (found, x::xs) = (found x) orelse find (found, xs)
,并使用它:
fun findItemHelper (name, ns) = find (fn (n, _) => name = n, ns)
fun findItem (name, nss) = find (fn ns => findItemHelper (name, ns), nss)