Fixing Language Extensions

It isn't rocket science

Posted on 2020-04-10

I do have a beef with Haskell language extensions though. But first a digression.

Where to place extensions

Some people like to declare a liberal set of language extensions in the cabal file and thereby be spared writing them at the head of modules. I accept that opinions will differ on this practice but I would not advocate it for these reasons.

  1. Each source module now needs the cabal file to make sense.

  2. Copying a source module between code bases needs to be done with care and could lead to silent breakage.

  3. With the extensive collection of Haskell language extensions I would really like to know just what is required for each source module that I read. The extensions provide me with a really useful handle to find the documentation when necessary.

To me (and I am not alone) throwing the switches on long list of extensions in the cabal file is taking liberties and weakens the case for having them.

That said, as this post atests, I am not wild about the status quo either, but feel there is much we could be doing to reduce this temptation (including publishing Haskell 2020.

Fixing In-Source-File Extensions

I really, really want to write:

{-# LANGUAGE
      DeriveAnyClass
      DerivingVia
      OverloadedStrings
      RecordWildCards
#-}

but I am forced to write (with obvious variants),

{-# LANGUAGE
      DeriveAnyClass
      , DerivingVia
      , OverloadedStrings
      , RecordWildCards
#-}

so I actually mostly write,

{-# LANGUAGE DeriveAnyClass             #-}
{-# LANGUAGE DerivingVia                #-}
{-# LANGUAGE OverloadedStrings          #-}
{-# LANGUAGE RecordWildCards            #-}

which is really unfortunate, and I suspect unnecessary.

Background

I tend to build up extensions as I write the module, and often have the relevant extension staring at me in a nearby module. It is useful to be able just transfer the relevant line.

I also like to keep them in a standard order, lexicographical order, which is really easy to do if every line is laid out uniformly regardless of where it appears in the list.

All of this gets broken when the compiler insists on comma separators.

The option of preceding the first extension with a comma fix this.

{-# LANGUAGE
      , DeriveAnyClass
      , DerivingVia
      , OverloadedStrings
      , RecordWildCards
#-}

but why not make them all optional?

Got an issue with any of this? Please share or drop me a line (see below).