Hi, Using LambdaCase extension, you can write something quite elegant: {-# LANGUAGE LambdaCase #-} f :: Int -> IO String f x = getDBRecord x >>= \case dbOutput | null dbOutput -> return "no db record" | otherwise -> return "we got some db records" Or better: f :: Int -> IO String f x = getDBRecord x >>= \case [] -> return "no db record" _ -> return "we got some db records" But you can also use another function for the pure part: recordMsg :: [a] -> String recordMsg [] = "no db record" recordMsg _ = "we got some db records" f :: Int -> IO String f = fmap recordMsg . getDBRecord Regards, Sylvain 2015-01-15 12:51 GMT+01:00 Miro Karpis <miroslav.karpis@gmail.com>:
Hi,
please is there a way to have guards with 'where' that communicates with IO? Or is there some other more elegant way? I can do this with classic if/else,...but I just find it nicer with guards.
I have something like this (just an example):
f :: Int -> IO String f x | null dbOutput = return "no db record" | otherwise = return "we got some db records" where dbOutput = getDBRecord x
getDBRecord :: Int -> IO [Int] getDBRecord recordId = do putStrLn $ "checking dbRecord" ++ show recordId --getting data from DB return [1,2]
problem is that db dbOutput is IO and the guard check does not like it:
Couldn't match expected type ‘[a0]’ with actual type ‘IO [Int]’ In the first argument of ‘null’, namely ‘dbOutput’ In the expression: null dbOutput
Cheers, Miro
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