loops - 可以cl :loop use a custom sum function?

Question

我可以为循环指定自定义求和函数(例如，vector-add)吗？

我想做这样的事情:

(loop for vec in '((1 2) (3 4)) sum vec)
;=> (4 6)

Answer 1

这是一个有趣的问题。如果你能做到这一点就好了，但简短的回答是“不”。根据6.1.3 Value Accumulation Clauses来自 HyperSpec:

The sum construct forms a cumulative sum of the successive primary values of the supplied form at each iteration. The argument var is used to accumulate the sum; if var is supplied, loop does not return the final sum automatically. The var argument is bound as if by the construct with to a zero of the appropriate type. Subsequent values (including any necessary coercions) are computed as if by the function +. If into var is used, a type can be supplied for var with the type-spec argument; the consequences are unspecified if a nonnumeric type is supplied. If there is no into variable, the optional type-spec argument applies to the internal variable that is keeping the sum. The default type is implementation-dependent; but it must be a supertype of type number.

您的实际用例是这么简单(遍历列表并计算它们的向量和)还是更复杂？如果就这么简单，你可以用reduce做你想做的事。 .它看起来更像或不太像

(reduce 'vector-add '((1 2) (3 4)))

其中 vector-add 是您的自定义求和函数。如果您仍然需要使用 loop，您可以使用 for sum = ... then ... 获得类似于 reduce 的行为 loop 和一个显式的 finally (return sum)。首先，定义vector-add，

(defun vector-add (x y)
  (mapcar '+ x y))

(vector-add '(1 2) '(3 4))
;=> (4 6)

我们可以做到:

(loop
   for vec in '((1 2) (3 4) (5 6))
   for sum = vec then (vector-add sum vec)
   finally (return sum))
;=> (9 12)