feature: add asGuardedError

[hrajchert]

This function can work well with chainP or tryCatch

It can be used to make sure an error is what we say it is. It can be implemented as following

import { UnknownError } from '@ts-task/task';

type Guard <G> = (val: any) => val is G;

export function asGuardedError<E> (guard: Guard<E>) {
    return function (val: any): E | UnknownError {
        if (guard(val)) {
            return val;
        } else {
            return new UnknownError(val);
        }
    };
}

And used like this

import * as fs from '@ts-task/fs';
import { tryCatch, asGuardedError } from '@ts-task/utils';

function isSyntaxError (error: any): error is SyntaxError {
    return error instanceof SyntaxError;
}

const asSyntaxError = asGuardedError(isSyntaxError);

export const readConfig = (path: string) =>
    fs.readFile(path).pipe(
        map(buffer => buffer.toString()),
        tryCatch(JSON.parse, asSyntaxError),
    );