Types mismatch when using the Maybe monad + TypeScript

[ythecombinator]

Hey, thanks for the amazing work on this lib.

First of all, I'd like to point that this is more likely to be a mistake of mine than a real problem related to monet.js or TypeScript–then please point and I'll close the issue ASAP 😄 .

The problem I am facing is:

I have a GithubClient class which basically takes care of processing data fetched from the Github API. The data is fetched using axios–a HTTP client abstraction lib–and I ended up using monet.js due to its Maybe monad since I wanted to properly handle Nothing from responses.

The code goes something like this:

import axios, {
  AxiosRequestConfig,
  AxiosResponse,
  AxiosError,
  AxiosInstance
} from "axios";
import { Maybe } from 'Monet';

export class GithubClient {

  private config: AxiosRequestConfig;
  private client: AxiosInstance;

  constructor() {

    this.config = {
      method: 'get',
      baseURL: 'https://api.github.com',
      headers: { 'Authorization': `token ${process.env.GITHUB_TOKEN}` },
      //withCredentials: true
    };

    this.client = axios.create(this.config)
  }

  // ↓ ↓ ↓ focus here ↓ ↓ ↓

  public async getAvatarUrl(username: string) {

    try {
      const response: AxiosResponse = await this.client.get(`users/${username}`)
      return this.handleResponse(response)
    } catch (e) {
      this.handleError(e);
    }

  }

  private handleResponse (response: AxiosResponse): Maybe<string> {
    return Maybe.fromNull(response.data.avatar_url)
  }

  private handleError(error: AxiosError) {
    (error.response)
      ? ( console.log(error.response.data),
          console.log(error.response.status),
          console.log(error.response.headers) )
      : console.log('Error: ', error.message);
  }

}

But the compiler throws me:

src/Server.ts(31,28): error TS2345: Argument of type 'Maybe<string>' is not assignable to parameter of type 'String'.
  Property 'charAt' is missing in type 'Maybe<string>'.

I don't really know from where String is being inferred nor from where charAt comes–I couldn't even find this on monet.js source.

Any ideas?

[desfero]

@ythecombinator on my side this code transpiles correctly and getAvatarUrl method has correct type Promise<Maybe<string>>.
How do you use getAvatarUrl?

[ulfryk]

@ythecombinator - error message informs that somewhere in the code Maybe<string> returned (probably) from getAvatarUrl is passed to a function which expects string.

Maybe you can share code around line 31 from src/Server.ts file ?