Xuan Huang (黄玄) Huxpro

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Hi folks, I just wanna share my story on my first day trying Cabal/Stack.

So I was learning Haskell for half a semester but only live within ghci and runhaskell. But today I was thinking of playing with /write-you-a-haskell/chapter4/untyped so I pulled it down.

I was first trying to follow the README.md which asked me to do cabal run. It failed and gave me errors on the ambiguous usage of <>, I realized cabal is using my global GHC version and it might be some breaking changes there. (since it's Haskell)

I heard stack can solve this problem by having multiple GHC so I changed to looking into that. So I run stack build, see the new GHC being downloaded, but failed. The resolver of the project is 7.8.4 and the problem is ar permission errors on OS X 10.11 El Capitan with GHC 7.8.4 . I tried several workarounds posted in the issue page but none of them works.

Aft

OK, so today I was looking at Writer monad, a very basic Monad.

In its essence, it's just a Monad that outputs (e.g. log) something,
The "Variation three: Output" Monad from the original Monads for functional programming, 92 paper is very lean:

type M a = (Output, a)
type Output = String

	module Query = [%graphql
	{\|
	query {
	someFieldWithInterfaceType {
	id
	...on ImplA {
	a_only
	}
	...on ImplB {
	b_only

	(* SIMPLE FUNCTIONS OVER INTEGERS *)
	let multiple_of n d =
	n mod d = 0;;

	let integer_square_root n =
	int_of_float (sqrt (float n));;

	(* PRIME NUMBERS *)
	let rec gcd n m =
	match (n, m) with

	module type Set = sig
	type 'a t
	val empty : 'a t
	val isEmpty : 'a t -> bool
	end

	module ListSet : Set = struct
	type 'a t = 'a list
	let empty = []
	let isEmpty l = l == []

	{-# LANGUAGE
	GADTs,
	ExistentialQuantification
	#-}

	import Control.Monad
	import Data.IORef
	import Data.Sequence
	import Data.Foldable

	import 'package:flutter/material.dart';

	/// A TextField that maintain selection state like React Native
	class TextInput extends StatefulWidget {

	TextInput({@required this.decoration, @required this.text, @required this.onChanged});

	final String text;
	final InputDecoration decoration;
	final Function onChanged;

	// 试了两种方式模拟：

	class Super {
	constructor() {
	this.x = "super"
	}
	};

	// 这样的话也是 own property
	new Super().hasOwnProperty('x') // true