HListPP

A preprocessor for HList labelable labels

Latest on Hackage:

0.2.2

This package is not currently in any snapshots. If you're interested in using it, we recommend adding it to Stackage Nightly. Doing so will make builds more reliable, and allow stackage.org to host generated Haddocks.

BSD-3-Clause licensed and maintained by Adam Vogt

A preprocessor that replaces `longDescriptiveName with

hLens' (Label :: Label "longDescriptiveName")

Use with ghc flags -F -pgmF HListPP. A file using HListPP could start with something like

❴-# OPTIONS_GHC -F -pgmF HListPP #-❵ -- change braces to ascii (haddock doesn't allow literal comments in files)
❴-# LANGUAGE DataKinds #-❵
import Data.HList.CommonMain
import Control.Lens

Then the labels can then be used in expressions with types like:

`x :: Lens (Record S) (Record T) a b
`x :: Prism (Variant S) (Variant T) a b
`a .==. "x" .*.
   `b .==. "y" .*. emptyRecord :: Record [ Tagged "a" String, Tagged "b" String ]

where S is a type-level list containing a Tagged "x" a and T is a type-level list containing a Tagged "x" b.

Note that `labels is expanded to something that takes up about 26 more columns. To avoid issues with layout, put a newline after keywords that introduce layout, at least if there are `labels in the same row. In other words:

`character.`position.`x %= \ x -> let
     msg = "a really long message about x>0"
     pos = x - 5
   in if pos < 0
         then error msg
         else pos

Instead of

`character.`position.`x %= \ x -> let msg = "a really long message about x>0"
                                      pos = x - 5
                           in if pos < 0
                               then error msg
                               else pos