I would not say that the problem is with the guard check. The problem is with 'null'. It's type is Prelude> :t null null :: [a] -> Bool So, it expects a list of something, rather than an IO of something, whence the complaint. Dimitri On 15/01/15 09:51, Miro Karpis wrote:
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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