Safe Haskell	None
Language	Haskell2010

System.IO.Streams.Csv.Tutorial

Contents

Types representing to-do items and their state
Functions which use cassava-streams functions

Description

A simple tutorial on using the cassava-streams library to glue together cassava and io-streams.

Note: if you're reading this on Hackage or in Haddock then you should switch to source view with the "Source" link at the top of this page or open this file in your favorite text editor.

Synopsis

Types representing to-do items and their state

data Item Source #

A to-do item.

Constructors

Item
Fields title :: String Title. state :: TState State. time :: Maybe Double Seconds taken to complete.

Instances

Eq Item Source #
Methods (==) :: Item -> Item -> Bool # (/=) :: Item -> Item -> Bool #
Show Item Source #
Methods showsPrec :: Int -> Item -> ShowS # show :: Item -> String # showList :: [Item] -> ShowS #
FromNamedRecord Item Source #
Methods parseNamedRecord :: NamedRecord -> Parser Item #
ToNamedRecord Item Source #
Methods toNamedRecord :: Item -> NamedRecord #

data TState Source #

Possible states for a to-do item.

Constructors

Todo	Item needs to be completed.
Done	Item has been finished.

Instances

Eq TState Source #
Methods (==) :: TState -> TState -> Bool # (/=) :: TState -> TState -> Bool #
Show TState Source #
Methods showsPrec :: Int -> TState -> ShowS # show :: TState -> String # showList :: [TState] -> ShowS #
FromField TState Source #
Methods parseField :: Field -> Parser TState #
ToField TState Source #
Methods toField :: TState -> Field #

Functions which use cassava-streams functions

onlyTodo Source #

Arguments

:: Handle	Input handle where CSV data can be read.
-> Handle	Output handle where CSV data can be written.
-> IO ()

The onlyTodo function reads to-do Items from the given input handle (in CSV format) and writes them back to the output handle (also in CSV format), but only if the items are in the Todo state. In another words, the CSV data is filtered so that the output handle only receives to-do Items which haven't been completed.

The io-streams handleToInputStream function is used to create an InputStream ByteString stream from the given input handle.

That stream is then given to the cassava-streams function decodeStreamByName which converts the InputStream ByteString stream into an InputStream Item stream.

Notice that the cassava-streams function onlyValidRecords is used to transform the decoding stream into one that only produces valid records. Any records which fail type conversion (via FromNamedRecord or FromRecord) will not escape from onlyValidRecords but instead will throw an exception.

Finally the io-streams filter function is used to filter the input stream so that it only produces to-do items which haven't been completed.

markDone Source #

Arguments

:: String	Items with this title are marked as `Done`.
-> Handle	Input handle where CSV data can be read.
-> Handle	Output handle where CSV data can be written.
-> IO ()

The markDone function will read to-do items from the given input handle and mark any matching items as Done. All to-do items are written to the given output handle.