GHC/Typed holes

Type holes in GHC.

Type holes are a powerful feature in GHC that is inspired by Agda. But what are type holes, and how do they help us write code?

(This page was taken from a 'tutorial' I wrote of this feature on Reddit.)

Introduction

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Motivating example

Let us consider the definition of a Free monad:

data Free f a
  = Pure a
  | Free (f (Free f a))

A free monad coupled with an

Writing 'instance Monad (Free f)'

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Notes on parametricity and Agda

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