NoRecursion plugin

A GHC plugin to remove support for recursion

General recursion can be the cause of a lot of problems. This removes recursion from GHC, allowing you to guarantee you’re using other mechanisms, like recursion schemes.

usage

Add no-recursion to your build dependencies.

Add -fplugin NoRecursion to your GHC options. This can be done per-module with

{-# OPTIONS_GHC -fplugin NoRecursion #-}

Now, any recursion in that module will result in a compilation failure.

NB: This won’t prevent you from using recursive functions imported from other modules, but inlined definitions from other modules will be checked.

allowing some recursion

NoRecursion supports two source annotations: "Recursion" and "NoRecursion".

You can re-enable recursion for individual top-level names like

recDef :: a -> b
recDef = myRecDef
{-# ANN recDef "Recursion" #-}

Or you can re-enable recursion for an entire module with

{-# ANN module "Recursion" #-}

And then you can re-disable recursion for individual names with

nonRecDef :: a -> a
nonRecDef = id
{-# ANN nonRecDef "NoRecursion" #-}

If both '"Recursion"' and "NoRecursion" annotations exist on the same name (or module), it’s treated as NoRecursion.

NB: If multiple names are mutually recursive, then they must all have recursion enabled to avoid being flagged by the plugin.

ANN has some caveats:

• ANN isn’t allowed by Safe Haskell, so any module that uses it will be inferred as Unsafe.

• If you enable the OverloadedStrings language extension, you will have to specify the type in the annotation, like

  {-# ANN module "Recursion" :: String #-}
  

For more about how to use annotations, see the GHC User’s Guide.

comparisons

Other projects similar to this one, and how they differ.

WartRemover

WartRemover is a Scala linting tool. A Recursion wart was added in 2017, and I’ve been meaning to write this plugin ever since. It only took seven years to find a few hours to make it happen …