标签：eal blog span ant sse nes else -- cal
We refactor a function that uses try/catch to a single composed expression using Either. We then introduce the chain function to deal with nested Eithers resulting from two try/catch calls.
For example we have this code using try & catch:
const getPort = () => { try { const file = fs.readFileSync(‘config.json‘); const c = JSON.parse(file); return c.port; } catch(e) { return 3000; } }
And now, we want to use Either to rewirte the code:
// SETUP: fake fs //========== const fs = { readFileSync: name => { if(name === ‘config.json‘) { return JSON.stringify({port: 8888}) } else { throw(‘missing file!‘) } } } //================= const Right = x => ({ map: f => Right(f(x)), fold: (f, g) => g(x), toString: () => `Right(${x})` }); const Left = x => ({ map: f => Left(x), fold: (f, g) => f(x), toString: () => `Left(${x})` }); const fromNullable = x => x != null ? Right(x): Left(null); const tryCatch = f => { try { return Right(f()); } catch(e) { return Left(e); } } //========================= const getPort = () => tryCatch(() => fs.readFileSync(‘config.json‘)) .map(f => JSON.parse(f)) .fold( x => 3000, x => x.port); console.log(getPort(‘config.json‘))
We wrote the function ‘tryCatch‘, the idea is put the code need to be checked in try, when success call ‘Right‘, error, call ‘Left()‘.
tryCatch(() => fs.readFileSync(‘config.json‘))
Read the file, is success, will return file content. If not, then goes to set default 3000.
.fold( x => 3000, x => x.port);
It works, but we still miss one things, in the old code, the ‘JSON.parse‘ are also wrapped into try catch.
const getPort = () => tryCatch(() => fs.readFileSync(‘config.json‘)) //Right(‘{port:8888}‘) .map(f => tryCatach(() => JSON.parse(f))) //Right(Right({port:8888}))
But once we also wrap parseing code into tryCatch function, the return value is 2d-Right. So we want to flatten it.
const Right = x => ({ map: f => Right(f(x)), flatMap: f => f(x), fold: (f, g) => g(x), toString: () => `Right(${x})` }); const Left = x => ({ map: f => Left(x), flatMap: f => f(x), fold: (f, g) => f(x), toString: () => `Left(${x})` });
We add ‘flatMap‘, so instead of putting the value into Right() or Left(), we just return the value. Because we know the value passed in is already a Right or Left.
const getPort = () => tryCatch(() => fs.readFileSync(‘config.json‘)) //Right(‘{port:8888}‘) .flatMap(f => tryCatch(() => JSON.parse(f))) //Right({port:8888}) .fold( x => 3000, x => x.port);
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// SETUP: fake fs //========== const fs = { readFileSync: name => { if(name === ‘config.json‘) { return JSON.stringify({port: 8888}) } else { throw(‘missing file!‘) } } } //================= const Right = x => ({ map: f => Right(f(x)), flatMap: f => f(x), fold: (f, g) => g(x), toString: () => `Right(${x})` }); const Left = x => ({ map: f => Left(x), flatMap: f => f(x), fold: (f, g) => f(x), toString: () => `Left(${x})` }); const fromNullable = x => x != null ? Right(x): Left(null); const tryCatch = f => { try { return Right(f()); } catch(e) { return Left(e); } } //========================= const getPort = () => tryCatch(() => fs.readFileSync(‘config.json‘)) //Right({port:8888}) .flatMap(f => tryCatch(() => JSON.parse(f))) //Right(Right({port:8888})) .fold( x => 3000, x => x.port); console.log(getPort(‘config.json‘))
[JS Compose] 3. Use chain for composable error handling with nested Eithers (flatMap)
标签：eal blog span ant sse nes else -- cal
原文地址：http://www.cnblogs.com/Answer1215/p/6175746.html
