Haskell Errors Binding a CSV File to a Handle - csv

Question

So I was writing a small toybox of utility functions for CSV files and throughout testing the functions I'd been binding the file by hand with
table' <- parseCSVFromFile filepath
but (from Text.CSV)
parseCSVFromFile :: FilePath -> IO (Either parsec-3.1.9:Text.Parsec.Error.ParseError CSV)
and so I had to write a quick line to strip off this either error csv crap
stripBare csv = head $ rights $ flip (:) [] csv
and redefine it as table = stripBare table'. After this, list functions work on the csv files contents and life goes on
(Digression: Surprisingly there isn't a direct Either a b -> b function in Data.Either. So I used Data.Either.rights :: [Either a b] -> [b])
I wanted to do the work of undressing the csv type and binding it to a handle in one shot. Something like
table = stripBare $ table' <- parseCSVFromFile filepath
but this gives a parse error on (<-) saying I may be missing a do... then
table = stripBare $ do table' <- parseCSVFromFile filepath
yells at me saying that the last statement in a do block must be an expression.
What am I doing wrong?
As a separate curiosity, I saw here that
do notation in Haskell desugars in a pretty simple way.
do
x <- foo
e1
e2
...
turns into
foo >>= \x ->
do
e1
e2
I find this attractive and tried the following line which gave me a type error
*Toy> :type (parseCSVFromFile "~/.csv") >>= \x -> x
<interactive>:1:52: error:
* Couldn't match type `Either
parsec-3.1.9:Text.Parsec.Error.ParseError'
with `IO'
Expected type: IO CSV
Actual type: Either parsec-3.1.9:Text.Parsec.Error.ParseError CSV
* In the expression: x
In the second argument of `(>>=)', namely `\ x -> x'
In the expression:
(parseCSVFromFile "~/.csv") >>= \ x -> x

Answer 1

Code such as
head $ rights $ flip (:) [] csv
is dangerous. You are exploiting the partiality of head to hide the fact that csv might be a Left something. We can rewrite it as
= head $ rights $ (:) csv []
= head $ rights [csv]
= case csv of
Left _ -> error "Left found!"
Right x -> x
Usually it's better to handle the Left case directly in the do IO block. Something like: (pseudo code follows)
foo :: String -> IO ()
foo filepath = do
table' <- parseCSVFromFile filepath
case table' of
Left err -> do
putStrLn "Error in parsing CSV"
print err
moreErrorHandlingHere
Right table -> do
putStrLn "CSV loaded!"
use table