[mrkgnao]

Well, Servant isn't introductory Haskell material. My point was to show that the advanced type system features of modern Haskell are useful in the real world, for, e.g. building webapps that real people use and ship to production.

If you've picked up the stuff in LYAH, you're ready to learn what type operators are (that's where :> and friends come from, they're just things like Maybe but written infix, so they're probably defined as

data a :> b = ColonPointyThing a b

or something like that.) Servant then pattern-matches on these types, essentially. For instance, if I can handle an API that serves endpoint A, and one that serves endpoint B, I can handle an API that serves both:

    instance (Handler A, Handler B) => Handler (A :<|> B) where

       handle req = ...

That's the idea.

You'd hardly expect a beginner to pick up, I dunno, using React and Redux on a Webpack hot-reloadable setup on day 1 of "Javascript 101", but React is one of the best ways to sell modern web development (at least when I've been buying).

[dmitriid]

The problem is basically how to arrive at the "same level" in Haskell and in JS.

Basically: what does it take to define (or even to just understand) a type-level DSL in Haskell, and a sufficiently advanced library in Javascript.

I can take apart almost any JS library and see/understand how it works. How much type-foo do I need to understand Servant? Or any other sufficiently complex Haskell library (a friend of mine has struggled mightily with Yesod and gave up after a month or so).

[mrkgnao]

I'm pretty sure you'll be able to follow the Servant docs, which are quite nice:

https://haskell-servant.readthedocs.io/en/stable/

There are definite advantages, and if you're willing to put in as much work as one, say, unconsciously puts in while setting up JS frameworks, understanding libraries like this is definitely doable. (Without the dissertation.)

[dmitriid]

The docs are nice, but they complement the code :)

Basically to actually understand what's going on, I'll need to learn DataKinds (at the very least).

And just to implement something as simple as BasicAuth, this is what I have to start with (https://haskell-servant.readthedocs.io/en/stable/tutorial/Au...):

  {-# LANGUAGE DataKinds             #-}
  {-# LANGUAGE DeriveGeneric         #-}
  {-# LANGUAGE FlexibleContexts      #-}
  {-# LANGUAGE FlexibleInstances     #-}
  {-# LANGUAGE MultiParamTypeClasses #-}
  {-# LANGUAGE OverloadedStrings     #-}
  {-# LANGUAGE ScopedTypeVariables   #-}
  {-# LANGUAGE TypeFamilies          #-}
  {-# LANGUAGE TypeOperators         #-}
  {-# LANGUAGE UndecidableInstances  #-}

You rarely need anything beyond regular JS to understand the inner workings of most libraries.

[mrkgnao]

Servant isn't the "simplest" way to get BasicAuth: you get what you pay for.

You can just use one of the raw HTTP servers, like wai or something. They expose an interface not unlike the ones you'll find in, say, Go or C++ or Node or something.

Also:

  {-# LANGUAGE FlexibleContexts      #-}
  {-# LANGUAGE FlexibleInstances     #-}
  {-# LANGUAGE MultiParamTypeClasses #-}
  {-# LANGUAGE TypeOperators         #-}
  {-# LANGUAGE UndecidableInstances  #-}

These extensions only serve to lift a couple of restrictions that the compiler imposes because the original Haskell Report did. DataKinds and TypeFamilies are the real "new ideas" that you need to pick up after a book at the level of LYAH to understand Servant.

[dmitriid]

> Servant isn't the "simplest" way to get BasicAuth: you get what you pay for.

But that's not what I complained about ;)

It's funny how in a separate thread someone insists that I have to understand the whole concept of a monad, its three laws, read Wadler's paper etc.

In his thread however, it boils down to: oh, just memorise these lines of code, and just blindly copy-paste them wherever.

So, my complaint is: to actually understand what's going in the "simplest way to do BasicAuth" I need a whole lot more than just look at the code.

I need to understand why I need no less than ten (!) language extensions before I even begin to implement BasicAuth. And five of them just to workaround some Haskell limitation based on some report from 1998 (I'm guessing)? What happens when I move on to OAuth? What will I need then?

Also, "oh, you don't really need PhDs in type theory to use Servant" slowly descends to "oh, here are two things regarding types that you'll have to learn". On of them has no useable documentation except some comment on StackOverflow. The other one requires you to be well-versed in type theory.

Hence my complaint about "you need a PhD in type theory to work with Haskell".

[mrkgnao]

> "simplest way to do BasicAuth"

The simplest way to do BasicAuth in Haskell isn't Servant. I feel like you're intentionally misinterpreting what I'm saying.

As for resources, here's a great intro to type-level Haskell:

http://www.parsonsmatt.org/2017/04/26/basic_type_level_progr...

> For some reason, functions that operate on types are called type families.

That's the kind of thing you need to know.

I doubt the average JS user knows how V8 optimizes execution of code, or whatever. That's what your insistent complaints about type theory amounts to: none of the articles I'm linking to mention inference rules or type judgments or what-have-you. That's what all the research papers about Haskell are for, which you do not need to read to use this language. Even advanced Haskellers don't do type theory (category theory, maybe, not type theory). Only a couple of people working on the compiler do.

More DataKinds:

https://stackoverflow.com/questions/20558648/what-is-the-dat...

http://ponies.io/posts/2014-07-30-typelits.html

https://www.schoolofhaskell.com/user/k_bx/playing-with-datak...