From: Michael Orlitzky Date: Tue, 31 Dec 2013 16:37:23 +0000 (-0500) Subject: Add the FromXml type family. X-Git-Tag: 0.0.1~139 X-Git-Url: https://gitweb.michael.orlitzky.com/?a=commitdiff_plain;h=53d408ee8691bc123dd55eba64e2c00cd17c8f57;p=dead%2Fhtsn-import.git Add the FromXml type family. --- diff --git a/src/Xml.hs b/src/Xml.hs index 129c6e9..21a8068 100644 --- a/src/Xml.hs +++ b/src/Xml.hs @@ -1,6 +1,9 @@ +{-# LANGUAGE TypeFamilies #-} + -- | General XML stuff. -- module Xml ( + FromXml(..), parse_opts, pickle_unpickle ) where @@ -22,6 +25,28 @@ import Text.XML.HXT.Core ( xunpickleVal, yes ) + +-- | A typeclass for types which can be converted into an associated +-- XML type. The story behind this is long, but basically, we need +-- to different types for each XML thingie we're going to import: a +-- database type and an XML type. Both Groundhog and HXT are very +-- particular about the types that they can use, and there's no way +-- to reuse e.g. a type that HXT can pickle in Groundhog. So this +-- typeclass gives us a way to get the XML type from the Groundhog +-- type. +-- +-- At first there appears to be an equally-valid approach, getting the +-- Groundhog type from the XML one. But Groundhog won't use type family +-- instances, so here we are. +-- +class FromXml a where + -- | Each instance a must declare its associated XML type (Xml a) + type Xml a :: * + + -- | And provide a function for getting an (Xml a) out of an "a." + to_xml :: a -> Xml a + + -- | A list of options passed to 'readDocument' when we parse an XML -- document. We don't validate because the DTDs from TSN are -- wrong. As a result, we don't want to keep useless DTDs