Add Two Values Wrapped inside Option Monad - fp-ts

Question

If I have two Option monads containing numbers, how can I add them together whilst not exiting the monad?
import { fromNullable, pipe, chain, map } from 'fp-ts/lib/Option'
let c1 = fromNullable(10)
let c2 = fromNullable(20)
// This is where I'm stuck. I would expect c3 to be an Option<number> containing 30.
let c3 = pipe(c1, chain(map((x) => x + c2))
Thank you :-)

Answer 1

You should use a sequence:
const c1 = Option.some(10);
const c2 = Option.some(20);
assertEquals(Array.sequence(Option.option)([c1, c2]), Option.some([10, 20]));
Look at https://fsharpforfunandprofit.com/posts/elevated-world-4/#sequence for explainations.

Answer 2

You are missing some pipe indirection in the snippet above. This will work:
import { option } from "fp-ts";
import { pipe } from "fp-ts/function";
declare const c1: option.Option<number>;
declare const c2: option.Option<number>;
const c3 = pipe(
c1,
option.chain(c1 =>
pipe(
c2,
option.map(c2 => c1 + c2)
)
)
);
There are various alternatives depending on the context/usage. Here are a few
Using Do from fp-ts-contrib:
import { Do } from "fp-ts-contrib/lib/Do";
const c3b = Do(option.option)
.bind("c1", c1)
.bind("c2", c2)
.return(({ c1, c2 }) => c1 + c2);
Using sequenceS from the Apply module:
import { sequenceS } from "fp-ts/Apply";
const c3c = pipe(
sequenceS(option.option)({ c1, c2 }),
option.map(({ c1, c2 }) => c1 + c2)
);

Answer 3

I came up with the following.
import {pipe} from 'fp-ts/lib/function'
import { getFoldableComposition } from 'fp-ts/Foldable'
import { array } from 'fp-ts/Array'
import { option,fromNullable } from 'fp-ts/Option'
import { monoidSum } from 'fp-ts/Monoid'
const F = getFoldableComposition(array, option)
let c1 = fromNullable(10)
let c2 = fromNullable(20)
let c3 = fromNullable(undefined)
let c4 = fromNullable(undefined)
// returns 30
F.reduce([c1,c2],0, monoidSum.concat) //?
// alternatively, also returns 30
F.reduce([c1,c2],0, (i,j)=>i+j) //?
// The above unwarp the result though. To return as the monad need to lift the result
// returns some(30)
pipe(F.reduce([c1,c2],0, (i,j)=>i+j), fromNullable) //?
// returns some(0)
pipe(F.reduce([c3,c4],0, (i,j)=>i+j), fromNullable) //?