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October 22, 2019 10:41
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| // http://www.typescriptlang.org/play/?ssl=1&ssc=1&pln=64&pc=1#code/PTAEjgVQjdMCldF-FQHU0Fxyho+QFAlIN0VBrcoTu1AWaoBD-8gI36DCcoM+BgL2aCwmoELmgIW6DLfoCHmgkP+D6coPRmgAHKDsFg0DKCYDAXdGEDyyoA3lQIAMgMATJgGH-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-Do7wAx9D1ADAABV+TfE9clfZpjDZK5QBMYwmgZFpDH0a4+naTRQAXJCHycD8txAtobwAZnvFtTyfLM1wbW140Ta99wAFiYgNHz1Niu1yRsuM-OiyN-dpGP9VNAOpPdeNo78ZP4+S02g+jYLABDNVfZMA1CVCmnQlpMOw3D8MI4jSPIwzy0EqiHVAEp8VUm8AFYBIUqtV3rJwDVE3UX0vHi9wANl8rSnHbVjOxzQL9Ftd9JLUjofM0wN4qizzpI6aLsuSXKvN00B9L6RzmJLUzzMs4wcLwgiiLI+zEOq5y22ojB3PSm8AHZIzHUIVw7GcxJCoNLHC0D9wADmGlpWzGhLswbZLUuonjwPaIa3nHKclPm8rKrfJcnEnOqMOxKzmtstqyI6kbciu-LdsWg7lyO-cBtO57ltGidros27Guslq7KegyLv0N6gA | |
| // --- | |
| // 后面用函数为例 | |
| // 其他容器（数组乃至更复杂的结构）也是一样的道理 | |
| // 协变、逆变，讲的是 Box<base> 和 Box<sub> 哪个才是 box_base | |
| // Box<base> 是 box_base，就是协变 | |
| // Box<sub> 变成 box_base，就是逆变 | |
| // --- | |
| // 什么叫 base，谁 instance 多谁就是 base | |
| // 表现为，接收 base 的地方，可以填 sub | |
| // --- | |
| // 至于 kotlin 之类的什么 in/out | |
| // 可以结合函数的例子来理解，参数 in 逆变，返回值 out 协变 | |
| // --- | |
| type base = string | number | |
| type sub = string | |
| // 对返回值协变 co-variance | |
| type co_base = () => base | |
| type co_sub = () => sub | |
| // 对参数逆变 contra-variance | |
| type contra_base = (data: sub) => null | |
| type contra_sub = (data: base) => null | |
| // case2 | |
| type contra_base2 = (data: sub) => base | |
| type contra_sub2 = (data: base) => sub | |
| // in-variance | |
| type in_base_X = (data: base) => base | |
| type in_sub_X = (data: sub) => sub | |
| // --- | |
| const f1: co_base = () => 0 | |
| const f2: co_sub = () => '' | |
| const test1: co_base = f2 | |
| const test2: co_sub = f1 // Type 'co_base' is not assignable to type 'co_sub' | |
| // --- | |
| const f3: contra_base = (data: sub) => null | |
| const f4: contra_sub = (data: base) => null | |
| const test3: contra_base = f4 | |
| const test4: contra_sub = f3 // Type 'contra_base' is not assignable to type 'contra_sub' | |
| // --- | |
| const f5: contra_base2 = (data: sub): base => 0 | |
| const f6: contra_sub2 = (data: base): sub => '' | |
| const test5: contra_base2 = f6 | |
| const test6: contra_sub2 = f5 // Type 'contra_base2' is not assignable to type 'contra_sub2' | |
| // --- | |
| const f7: in_base_X = (data: base): base => 0 | |
| const f8: in_sub_X = (data: sub): sub => '' | |
| const test7: in_base_X = f8 // Type 'in_sub_X' is not assignable to type 'in_base_X' | |
| const test8: in_sub_X = f7 // Type 'in_base_X' is not assignable to type 'in_sub_X' |
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