{- |
Module      :  $Header$
Description :  Template language syntax tree.
Author      :  Nils 'bash0r' Jonsson
Copyright   :  (c) 2015 Nils 'bash0r' Jonsson
License     :  MIT

Maintainer  :  aka.bash0r@gmail.com
Stability   :  unstable
Portability :  non-portable (Portability is untested.)

This module contains the syntax tree for the templating language.
-}
module Headergen.Template
( KeyValue
, Dictionary

, VariableName
, Template (..)

, fillTemplate
) where


import Control.Applicative
import Control.Monad


-- | A key value pair.
type KeyValue   = (String, String)
-- | A list of key value pairs.
type Dictionary = [KeyValue]

-- | The name of a variable in a template.
type VariableName = String


-- | A template is used when 
data Template
  -- | A simple text element in the template.
  = TemplateText     String
  -- | A variable to substitute in later process in the template.
  | TemplateVariable VariableName
  -- | A sequence of several elements in the template.
  | TemplateSequence Template Template
  -- | The end of a template.
  | TemplateEOF
  deriving (Show, Eq)


fillTemplate :: Dictionary -> Template -> Either String String
fillTemplate dict (TemplateEOF                ) = Right ""
fillTemplate dict (TemplateVariable var       ) = case dict `find` var of
    Just v  -> Right v
    Nothing -> Left ("Variable " ++ var ++ " is not defined.")
fillTemplate dict (TemplateText     text      ) = Right text
fillTemplate dict (TemplateSequence left right) = do
  l <- fillTemplate dict left
  r <- fillTemplate dict right
  return (l ++ r) 


find ((key, value):xs) var =
  if key == var
     then Just value
     else find xs var
find []                _   =
  Nothing