Commits on Feb 4, 2020

1. (WIP) Distinguish variable-not-found-error from others …

   Problem
   ====
   
   - Handling `Maybe` values in the `Parser` monad is troublesome.
   - Even `envMaybe` mixes up variable-not-found-error with others,
     which makes the difference of `Nothing` and `Left String`
     obscure.
   
   Solution
   ====
   
   Change the error type of `Parser a` from just a `String` into
   a dedicated error ADT `ParseError`.
   
   NOTE
   ====
   
   - Fix dmjio#30 as a bonus!

   

   igrep committed Feb 4, 2020
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