包裹函数 – IO
最后更新于：2022-04-02 04:17:58
[TOC]
## IO
```
var IO = function(f) {
this.unsafePerformIO = f;
}
IO.of = function(x) {
return new IO(function() {
return x;
});
}
IO.prototype.map = function(f) {
return new IO(_.compose(f, this.unsafePerformIO ));
}
```
`IO`跟之前的 functor 不同的地方在于，它的`__value`总是一个函数
实例1
```
// io_window_ :: IO Window
var io_window = new IO(function(){ return window; });
io_window.map(function(win){ return win.innerWidth });
// IO(1430)
io_window.map(_.prop('location')).map(_.prop('href')).map(split('/'));
// IO(["http:", "", "localhost:8000", "blog", "posts"])
// $ :: String -> IO [DOM]
var $ = function(selector) {
return new IO(function(){ return document.querySelectorAll(selector); });
}
$('#myDiv').map(head).map(function(div){ return div.innerHTML; });
// IO('I am some inner html')
```
实例2
```
////// 纯代码库: lib/params.js ///////
// url :: IO String
var url = new IO(function() { return window.location.href; });
// toPairs = String -> [[String]]
var toPairs = compose(map(split('=')), split('&'));
// params :: String -> [[String]]
var params = compose(toPairs, last, split('?'));
// findParam :: String -> IO Maybe [String]
var findParam = function(key) {
return map(compose(Maybe.of, filter(compose(eq(key), head)), params), url);
};
```
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