This repository contains an implementation of lenses written and verified in Agda, that can be translated to Haskell using agda2hs. This project is a part of the Research Project at the Delft University of Technology, iteration 2022 Q4.

All Agda source files are contained in src/, all resulting Haskell files after translating with agda2hs are found in lib/. Within these directories, the library is structured as follows:

• Control/Lens.[agda|hs] - Definition of the Lens record type and its methods.
• Control/Lens/Record.[agda|hs] - Example definition of a record type and a lens that operates on it.
• Control/Lens/Tuple.[agda|hs] - Lenses one and two for operating on a 2-tuple's first and second field respectively.
• Control/Lens/List.[agda|hs] - A non-well-behaved ix i lens constructor for lenses that operate on list indices.
• Control/Lens/Vec.agda - A lens that operates on Agda's Vec type. Not translated to Haskell.

To use this repository it is necessary to have cabal, ghc and make installed. If you don't have these installed you can follow the instructions below, else you can continue on to here.

It is recommended that you install chocolately as a package manager that allows for installing software by using the built-in terminal in Windows. For the latest instruction on installing chocolately click here and use the individual installation method.

To install all prerequisites execute the following commands in your terminal:

choco install ghc cabal make
refreshenv

Now you can go ahead and install the dependencies.

This template relies on agda2hs, Agda, and agda-stdlib. To build them from source, do the following:

git clone https://github.com/flupe/verification-template
cd verification-template
cabal install Agda    # If there are conflicting dependencies for base use the following flag: --allow-newer=base
cabal install agda2hs # If there are conflicting dependencies for base use the following flag: --allow-newer=base

Building Agda may take a while.

Follow the standard library's instructions to install it.

In order to use the Haskell prelude of agda2hs from your Agda code, you also need to tell Agda where to locate the library.

Clone agda2hs:

git clone https://github.com/agda/agda2hs

Inside the file ~/.agda/libraries, add the following line:

/your/path/to/agda2hs/agda2hs.agda-lib
/your/path/to/agda-stdlib/standard-library.agda-lib

First off execute the following command:

(test-path -path $home\AppData\Roaming\agda\libraries -pathtype Leaf) ? (echo "File not created, it already exists") : (new-item -path $home\AppData\Roaming\agda\libraries)

Either add the following line to C:\Users\<USER>\AppData\Roaming\agda\libraries or alternatively when using powershell $home\AppData\Roaming\agda\libraries,

</your/path/to/agda2hs/agda2hs.agda-lib>
</your/path/to/agda-stdlib/standard-library.agda-lib>

Or run the following commands in powershell:

add-content $home\AppData\Roaming\agda\libraries "`n</your/path/to/agda2hs/agda2hs.agda-lib>"
add-content $home\AppData\Roaming\agda\libraries "`n</your/path/to/agda2hs/standard-library.agda-lib>"

You will have to create this file if it does not exist.

You should be good to go. Open any file in the src/ directory inside your IDE of choice and you should be able to use the Haskell prelude in your code without any issue.

Running make at the root of the project will:

• compile Everything.agda using agda2hs. Don't forget to import your agda files in Everything.agda to have them compile to Haskell automatically.
• compile the Haskell library generated from the Agda files in lib/.
• comile the demo Haskell executable in demo/

To run the demo executable, just launch cabal run demo.

To test out your library in a REPL, use cabal repl project.