带有选项和链的 fp-ts 管道不起作用

Question

I have this sample code:

import {none, some, chain} from 'fp-ts/lib/Option';
import {pipe} from 'fp-ts/lib/pipeable';

const f1 = (input: string) => {
    return some(input + " f1")
};
const f2 = (input: string) => {
    return some(input + "f2")
};
const f3 = (input: string) => {
    return none;
};
const f4 = (input: string) => {
    return some(input + "f4");
};

const result = pipe(
    f1,
    chain(f2),
    chain(f3),
    chain(f4),
)("X");

console.log("result", result);

And I am getting this compile time error

Argument of type '(input: string) => Option<string>' is not assignable to parameter of type 'Option<string>'.
  Type '(input: string) => Option<string>' is missing the following properties from type 'Some<string>': _tag, value

18     f1,
       ~~

  src/index.ts:18:5
    18     f1,
           ~~
    Did you mean to call this expression?

What is wrong with my code?

I expect f1 and f2 to run and other function not because of none returning in f3 and at the end the output to be Some "X f1 f2"

Answer 1

The fp-ts pipe function expects the initial value "X" as first argument to facilitate TypeScript left to right generic inference.

So in contrast to other fp-libraries, where the initial value is passed in a curried manner, you create the pipe as follows:

const result = pipe(
  "X", // here is initial argument
  f1,
  chain(f2),
  chain(f3),
  chain(f4)
); // type: Option<string>, actual value is None

The return value will be None - once an option is None , it will stay None , when you chain over it (implementation here ):

chain((n: number) => some(n*2))(none) // stays None

---

Edit:

flow (equivalent of other libraries' pipe ) is an alternative, that behaves in the way you want it in the example:

import { flow } from "fp-ts/lib/function";

const result3 = flow(
  f1,
  chain(f2),
  chain(f3),
  chain(f4)
)("X")

There may arise type problems. For example, it is necessary to have the function parameter types of the first function ( f1 ) annotated with explicit types. Also consider, that pipe is seen as the new "blessed way" for compositions by the maintainers.