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@dypsilon /reader.js
Last active Sep 29, 2018

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Example usage of the reader monad.
/**
* This short program will encrypt the user password
* and insert a new record into a mock database.
*/
const Reader = require('fantasy-readers');
const R = require('ramda');
const crypto = require('crypto');
// our mock database
const database = [
{ email: 'user@example.org', password: 'e0538fd8f022bb3b139d72cf12766cb0e31690ff' },
{ email: 'admin@example.org', password: '42c4fbf6fec201c66b82c97833b08d936d2cd526' }
]
// creates a statefull database connection
const connectTo = (db) => {
return {
insert: (doc) => db.push(doc),
get: (i) => db[i],
delete: (i) => db.splice(i, 1),
list: () => db
}
}
// some utility functions
const encrypt = (i) => crypto.createHash('sha1').update(i).digest('hex');
const encPassword = R.evolve({password: encrypt})
const getInput = () => ({ email: 'new@example.org', password: 'secret' });
// this is how you access the db connection inside the reader
const save = (user) => {
return Reader.ask.map((db) => {
db.insert(user);
return db.list();
});
}
// the body of the program
const prog = R.pipe(
Reader.of,
R.map(encPassword),
R.chain(save)
);
// this is our db connection now
const dbCon = connectTo(database);
// this is how you pass the db connection in
const result = prog(getInput()).run(dbCon);
// show the output
console.log(result);
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mko-io Nov 29, 2016

Beautiful

mko-io commented Nov 29, 2016

Beautiful

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danielo515 Jul 6, 2018

This is nice, but quite simplistic. I would love to see an implementation involving other monads for async operations (which DB interactions usually are)

danielo515 commented Jul 6, 2018

This is nice, but quite simplistic. I would love to see an implementation involving other monads for async operations (which DB interactions usually are)

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danny-andrews Jul 21, 2018

@danielo515 I ran into the same issue when I was trying to use a Reader Monad for dependency injection. I was having to do a bunch of nested 'map's to get to the value inside the promise. But then I learned about monad transformers! I wrote one called ReaderPromise which seems to do the trick. https://github.com/danny-andrews/circleci-weigh-in/blob/master/src/shared/reader-promise.js

danny-andrews commented Jul 21, 2018

@danielo515 I ran into the same issue when I was trying to use a Reader Monad for dependency injection. I was having to do a bunch of nested 'map's to get to the value inside the promise. But then I learned about monad transformers! I wrote one called ReaderPromise which seems to do the trick. https://github.com/danny-andrews/circleci-weigh-in/blob/master/src/shared/reader-promise.js

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