a mighty handy extension not universally appreciated


Haskell records get a bad rap but I must say I have got so used to them that I no longer see the warts and RecordWildCards certainly help, but they are not without problems.

A quick reminder form some code used to generate this page. Given the following definition,

data ExtensionReview =
    { _er_id      :: ExtensionID
    , _er_date    :: Date
    , _er_home    :: URL
    , _er_stars   :: Stars

the pattern form ExtensionReview{..} can be used to make available the fields of the record under their own name.

instance Buildable ExtensionReview where
  build er@ExtensionReview{..} =
      "<div class=cd-er-row>"
        +|exnm _er_id   |+"\n"
        +|date _er_date |+"\n"
        +|home _er_home |+"\n"
        +|strs _er_stars|+

The problem are twofold:

  • the record fields are normally used as projection functions to extract the fields from values of the type, which are no longer are available inside the scope of this pattern construction;

  • In addition to identifiers flipping their meaning inside the scope of the pattern, Haskellers are not used to a bunch of identifiers springing into scope inside an expression. It is just not the way Haskell rolls normally.

So RecordWildCards are considered a bit icky by some Haskellers and maybe they are. But I will tell you what, they are just way too useful to allow such scruples get in the way of their use. My advice to anyone avoiding them is to park your scruples, bite down and give them a try. You might find them just too useful.

(I only consider RecordWildCards used in patterns here — see Dmitrii Kovanikov’s blog post for a discussion that includes their use to construct records in expressions.)

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