- pour utiliser les lens, voici ce que fait cabal:

Resolving dependencies...

Downloading transformers-compat-0.1.1.1...

Downloading tagged-0.7.1...

Configuring tagged-0.7.1...

Configuring transformers-compat-0.1.1.1...

Building transformers-compat-0.1.1.1...

Building tagged-0.7.1...

Installed transformers-compat-0.1.1.1

Installed tagged-0.7.1

Downloading contravariant-0.4.4...

Downloading distributive-0.4.3.1...

Configuring contravariant-0.4.4...

Configuring distributive-0.4.3.1...

Downloading reflection-1.4...

Configuring reflection-1.4...

Building contravariant-0.4.4...

Building reflection-1.4...

Installed contravariant-0.4.4

Building distributive-0.4.3.1...

Installed reflection-1.4

Installed distributive-0.4.3.1

Downloading comonad-4.0.1...

Configuring comonad-4.0.1...

Building comonad-4.0.1...

Installed comonad-4.0.1

Downloading semigroupoids-4.0.1...

Configuring semigroupoids-4.0.1...

Building semigroupoids-4.0.1...

Installed semigroupoids-4.0.1

Downloading bifunctors-4.1.1...

Downloading profunctors-4.0.3...

Configuring bifunctors-4.1.1...

Configuring profunctors-4.0.3...

Building bifunctors-4.1.1...

Building profunctors-4.0.3...

Installed profunctors-4.0.3

Installed bifunctors-4.1.1

Downloading free-4.6.1...

Configuring free-4.6.1...

Building free-4.6.1...

Installed free-4.6.1

Downloading lens-4.1.2...

Configuring lens-4.1.2...

Building lens-4.1.2...

Installed lens-4.1.2

Est-ce vraiment nécessaire ?

Je pensais qu'on ne pouvait pas l'éviter, je vais voir, meci.