[英]Problem writing mutually recursive function in F#
我正在翻译来自Little Mler的对此数据类型进行操作的函数
type sexp<'T> =
An_Atom of 'T
| A_slist of slist<'T>
and
slist<'T> =
Empty
| Scons of sexp<'T> * slist<'T>
功能
// occurs_in_slist : aVal slist -> int
// checks the number of occurrence for aVal in slist
let rec occurs_in_slist =
function
_, Empty-> 0
| (aVal : 'T), Scons(aSexp, (aSlist : 'T)) ->
occurs_in_sexp (aVal, aSexp) + occurs_in_slist (aVal, aSlist)
and
aVal, An_Atom (bVal) -> if (aVal = bVal) then 1 else 0
| (aVal , A_slist(aSlist)) -> occurs_in_slist (aval, aSlist)
但是,第二个功能却出现此错误
error FS0010: Unexpected symbol '->' in binding. Expected '=' or other token.
在函数定义中,您使用了and
关键字定义了一组相互递归的函数,但是仅给第一个函数指定了名称。 它的后预计其它功能的名称and
这就是为什么你得到错误。 不幸的是,您忽略了这一点。
我相信这是您想要做的:
let rec occurs_in_slist = function
| _ , Empty -> 0
| aVal : 'T, Scons(aSexp, aSlist : slist<'T>) ->
occurs_in_sexp (aVal, aSexp) + occurs_in_slist (aVal, aSlist)
and occurs_in_sexp = function
| aVal : 'T, An_Atom(bVal) -> if (aVal = bVal) then 1 else 0
| aVal , A_slist(aSlist) -> occurs_in_slist (aVal, aSlist)
尽管我觉得这里更合适的返回类型应该是bool
。
let rec occurs_in_slist = function
| _ , Empty -> false
| aVal : 'T, Scons(aSexp, aSlist : slist<'T>) ->
occurs_in_sexp (aVal, aSexp) || occurs_in_slist (aVal, aSlist)
and occurs_in_sexp = function
| aVal : 'T, An_Atom(bVal) -> aVal = bVal
| aVal , A_slist(aSlist) -> occurs_in_slist (aVal, aSlist)
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