为什么promise不是Scheme中的数据类型？
在Scheme中delay返回的对象是“一个promise”，但是promise不被认为是一种类型（所以没有promise？过程，并且它没有被列为R5RS 或 R6RS 中的一种类型）。
为什么会这样？有强烈的理由吗？例如，对我来说，做类似 (if (promise? x) (force x) x) 的事情似乎很自然。 （我发现有些实现会让我强制执行非承诺，而其他实现则不会）。另外，如果我可以将某些内容存储在变量中并将其传递，我觉得它应该有一个类型。
The object returned by delay in Scheme is "a promise", but promises are not considered to be a type (so there is no promise? procedure, and it's not listed as a type in R5RS or R6RS).
Is there a strong reson why this is so? It would seem quite natural to me to do something like (if (promise? x) (force x) x), for example. (And I see that some implementations will let me force non-promises, and others will not). Also, if I can store something in a variale and pass it around, I feel like it should have a type.
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不可能有那么有力的理由，因为 MIT/GNU 方案，定义了一个
promise?函数。There can't be that strong a reason, since MIT/GNU scheme, defines a
promise?function.我认为它可以更优化地实现延迟/强制。事实上，强制值可以被记忆（这样一个 Promise 只被强制一次，结果值会在后续的 Force 调用中返回）模糊了 Promise 和它的结果值之间的区别。如果你有
promise？你就不能在任何需要它的地方用它的值来代替强制的promise。因此，根据实现的不同，promise 可能与任何其他 Scheme 值无法区分。I think it allows for a more optimized implementation of
delay/force. The fact that the forced value can be memoized (so that a promise is really forced only once and the resulting value is returned on subsequentforcecalls) blurs the distinction between a promise and its resulting value. If you havepromise?you cannot substitute a forced promise by its value everywhere it is needed. Therefore, depending on the implementation, a promise can be indistinguishable from any other Scheme value.