flowtype - 带有联合参数的函数间接导致联合成员出现莫名其妙的错误

Question

在下面的代码块中，流错误仅在函数存在时发生在OuterY和OuterZ类型定义上。getInnerValues

这些错误抱怨"Y"并且"Z"与"X". 例如：“字符串文字Y与字符串文字不兼容X。” .

/* @flow */

type Inner<T> = { value: T };

type OuterX = { inner: Array<Inner<"X">> };
type OuterY = { inner: Array<Inner<"Y">> };
type OuterZ = { inner: Array<Inner<"Z">> };

type Outer = OuterX | OuterY | OuterZ;

// If the next line is present, errors occur on
// lines 6 and 7 complaining that "Y" and "Z" are
// incompatible with "X". When the next line is
// commented out, the errors go away. Why??
const getInnerValues = (outer: Outer) => outer.inner.map(inner => inner.value);

为什么会这样？

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标签： flowtype

Answer 1

Flow 没有意识到对于所有可能的情况都存在inner类型属性。解决此问题的一种方法是键入函数以接受具有预期类型的​​对象：{value: string}Outer

（试试）

/* @flow */

type Inner<T> = { value: T };

type OuterX = { inner: Array<Inner<"X">> };
type OuterY = { inner: Array<Inner<"Y">> };
type OuterZ = { inner: Array<Inner<"Z">> };

type Outer = OuterX | OuterY | OuterZ;

// no errors
const getInnerValues = (outer: {inner: Array<{value: string}>}) =>
    outer.inner.map(inner => inner.value);

另一种方法（可能是更好的方法）是重新定义Outer为接受类型参数的类型。然后，您可以通用地键入您的getInnerValues函数以接受通用Outer实例：

（试试）

/* @flow */

type Inner<T> = { value: T };

type OuterX = { inner: Array<Inner<"X">> };
type OuterY = { inner: Array<Inner<"Y">> };
type OuterZ = { inner: Array<Inner<"Z">> };

type Outer<T> = {
  inner: Array<Inner<T>>               
}

// no errors
const getInnerValues = <T>(outer: Outer<T>) => outer.inner.map(inner => inner.value);