Arnau Abella monadplus

Rewrite Rules

{-# RULES "map/map" forall f g xs. map f (map g xs) = map (f.g) xs #-}

Use the debug flag -ddump-simpl-stats to see what rules fired.
Use -ddump-rule-firings to show individual rule firing.
Use -ddump-rule-rewrite to show the code before and after rewrite.

Is having an algebraic effect important?

Given a monadic operation

op :: forall a. (M a, ..., M a) -> M a

then it is algebraic if it satisfies the following law:

Why logging is an effect we want to track

P. Oscar Boykin>
I love functional programming. I’m still very skeptical that logging is an effect we want to track. 
Logging is for observability. Removing all logging shouldn’t change the semantics of the program. 
By using an effect we change the types of many logged functions.

GHC Architecture

GHC itself is written in Haskell, but the runtime system for Haskell, essential to run programs, is written in C and C--.

Front end

GHC's frontend - incorporating the lexer, parser and typechecker—is designed to preserve as much information about the source language as possible until after type inference is complete, toward the goal of providing clear error messages to users.

The Handle Pattern

This pattern is design for qualified import this is why all types are called the same (Database.Handle, Socket.Handle, Database.Config, Secket.Config, ...)

A database Handle

What you need:

A type called Handle

Cachix

User

Add the cache:

$ cachix use monadplus

Money in Haskell

Floats (dont)

>>> 0.10 + 0.70 -- Fractional a
0.7999999999999999
>>> (0.1 + 0.7) * 2 -- Fractional a
1.5999999999999999
&gt;&gt;&gt; 0.10 + 0.60 :: Float

Optics: cheatsheet

Lens
Fold
Indexable Structures
Prism
Iso
Indexed Optics
Optics and Monads
Classy Lenses

	{-# LANGUAGE FlexibleContexts #-}

	-- Credit for this code to: https://github.com/pva701
	module Task0
	( f
	, f1
	, f2
	)where

	import Control.Monad.Except (throwError, runExceptT, catchError, MonadError, runExcept)

	-- \| Generic transformation for map to a total function 'key' -> 'value.
	--
	-- It fails if the given 'Data.Map.Strict' does not contains all the options
	-- for the given 'key'.
	--
	-- The 'key' is expected to be a unary coproduct with an Ord instance.
	class GGuaranteed gEnum where
	gguarantee :: Map (gEnum key) value
	-> Either (gEnum key) (gEnum key -> value)