How to restrict function parameters types base on if the index of parameter is even or odd in typescript?

Question

What I'm looking for is to have a function with infinite number of parameters and each parameter type is based on if its index is even or odd.

A contrived example:

flow(isMachineReady(), 'and', isWaterHot(), 'or', isMilkHot(), 'then', serveCoffee())

Odd arguments: () => boolean Even arguments: 'and' | 'or' | 'then'

So if the function was used like this I want to get error for the second argument type:

flow(isMachineReady(), **isWaterHot()**, 'or', isMilkHot(), 'then', serveCoffee())

Here is what I tried before but didn't work

type FlowArgCouple = [operand: () => boolean, operator: 'and' | 'or' | 'then']

function flow(...args: [...FlowArgCouple][]){
   ...
}

Dimava · Accepted Answer · 2022-12-23 11:34:05Z

Playground
But srsly consider some other structure

import { F } from "ts-toolbelt"

// [T1] or [T1, T2, T1] or [T1, T2, T1, T2, T1]
type ToOddEvenOdd<A extends any[], T1, T2, P extends any[]> =
    | A extends [] ? [...P, T1]
    : A extends [infer F extends T1] ? [...P, F]
    : A extends [infer F extends T1, infer S extends T2, ...infer L] ? ToOddEvenOdd<L, T1, T2, [...P, F, S]>
    : A extends [infer F, infer S, ...infer L] ? ToOddEvenOdd<L, T1, T2, [...P, T1, T2]>
    : [[A]]


type ODD = () => boolean
type EVEN = 'and' | 'or' | 'then'
function g<A extends any[]>(
    ...args:
        A extends ToOddEvenOdd<A, ODD, EVEN, []> ? A
        : 
        F.NoInfer< ToOddEvenOdd<A, ODD, EVEN, []> >
) { }

g(() => true)
g(() => true, 'and', () => false)
g(() => true, () => false)
g(() => true, 'and')
g(() => true, () => false, () => false)
g(() => true, 'and', 'and')

hassan salehi · Accepted Answer · 2022-12-26 14:48:26Z

Nice one Dimava, the solution you have works. Although for some reason it takes time for typescript to find out the error.

I find out another solution which also works well. But same issue, not always gives you the error as soon as it's there. But I add this answer as well, in case anyone else running to the same question can use it:

Playgournd

type Operand = () => boolean
type Operator = 'and' | 'or' | 'then'

type ValidRepeatedTuple<
    T extends readonly unknown[],
    U extends readonly unknown[]
> = U extends readonly []
    ? U
    : readonly [...T, ...(U extends readonly [...T, ...infer R] ? ValidRepeatedTuple<T, R> : [])]

type FlowArgs<U extends readonly unknown[]> = [
    ...ValidRepeatedTuple<[Operand, Operator], U>,
    Operand
]

function flow<U extends unknown[]>(...args: FlowArgs<U>){
}

flow(() => true)
flow(() => true, 'and', () => false)
flow(() => true, () => false)
flow(() => true, 'and')
flow(() => true, () => false, () => false)
flow(() => true, 'and', 'and')

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