add column indexing function

colonnade/src/Colonnade/Decoding.hs

@@ -23,9 +23,12 @@ headless f = DecodingAp Headless f (DecodingPure id)
 headed :: content -> (content -> Either String a) -> Decoding Headed content a
 headed h f = DecodingAp (Headed h) f (DecodingPure id)
 
+-- | Maps over a 'Decoding' that expects headers, converting these
+--   expected headers into the indices of the columns that they
+--   correspond to.
 headedToIndexed :: forall content a. Eq content
-                => Vector content
-                -> Decoding Headed content a
+                => Vector content -- ^ Headers in the source document
+                -> Decoding Headed content a -- ^ Decoding that contains expected headers
                 -> Either (HeadingError content) (Decoding Indexed content a)
 headedToIndexed v = go
   where

colonnade/src/Colonnade/Types.hs

@@ -14,6 +14,8 @@ module Colonnade.Types
 import Data.Vector (Vector)
 import Data.Functor.Contravariant (Contravariant(..))
 import Data.Functor.Contravariant.Divisible (Divisible(..))
+import Control.Exception (Exception)
+import Data.Typeable (Typeable)
 import qualified Data.Vector as Vector
 
 -- | Isomorphic to 'Identity'
@@ -33,6 +35,8 @@ data HeadingError content = HeadingError
   , headingErrorDuplicate :: Vector (content,Int) -- ^ headers that occurred more than once
   } deriving (Show,Read)
 
+instance (Show content, Typeable content) => Exception (HeadingError content)
+
 instance Monoid (HeadingError content) where
   mempty = HeadingError Vector.empty Vector.empty
   mappend (HeadingError a1 b1) (HeadingError a2 b2) = HeadingError