Synopsis

While trying to solve #4 for the Advent of Code 2018, I ran into a strange error when using the literal form for a regex:

Solve.hs:63:46: parse error on input ‘{’

The offending line was:

  where regex = [re|${y}([0-9]{4})-${m}([0-9]{2})-${d}([0-9]{2})|]

The (at least to me) strange thing was that this line was perfectly fine when I put it into a separate source file. Turns out that the compiler directive

{-# LANGUAGE QuasiQuotes                      #-}

has to come before the module declaration in the source file. Swapping the ordering of the compiler directive and the module declaration solved the problem.