Zehao Chen zehaochen19

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Yoneda Intuition from Humble Beginnings

Let's explore the Yoneda lemma. You don't need to be an advanced Haskeller to understand this. In fact I claim you will understand the first section fine if you're comfortable with map/fmap and id.

I am not out to motivate it, but we will explore Yoneda at the level of terms and at the level of types.

The Problem

Defining records in Haskell causes accessor functions for the record's fields to be defined. There is no seperate namespace for these accessor functions.

The Goal

Be able to

use records in Haskell, which share field names.
use lenses for accessing these fields

	{-# LANGUAGE GeneralizedNewtypeDeriving #-}

	module Language.HigherRank.Main
	( Expr(..)
	, EVar(..)
	, Type(..)
	, TVar(..)
	, TEVar(..)
	, runInfer
	) where

	This is my short-ish tutorial on how to implement closures in
	a simple functional language: Foo.

	First, some boilerplate.

	> {-# LANGUAGE DeriveFunctor, TypeFamilies #-}
	> import Control.Applicative
	> import Control.Monad.Gen
	> import Control.Monad.Writer
	> import Data.Functor.Foldable