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
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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