hw7: starting materials

[yorgey.git] / hw7 / StringBuffer.hs

diff --git a/hw7/StringBuffer.hs b/hw7/StringBuffer.hs
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+{-# LANGUAGE FlexibleInstances, TypeSynonymInstances #-}
+module StringBuffer where
+
+import Data.Monoid
+
+import Buffer
+
+instance Buffer String where
+  toString     = id
+  fromString   = id
+  line n b     = safeIndex n (lines b)
+  replaceLine n l b = unlines . uncurry replaceLine' . splitAt n . lines $ b
+      where replaceLine' pre [] = pre
+            replaceLine' pre (_:ls) = pre ++ l:ls
+  numLines     = length . lines
+  value        = length . words
+
+safeIndex :: Int -> [a] -> Maybe a
+safeIndex n _ | n < 0 = Nothing
+safeIndex _ []        = Nothing
+safeIndex 0 (x:_)     = Just x
+safeIndex n (_:xs)    = safeIndex (n-1) xs
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