[Haskell-beginners] help with IO guards

[Dimitri DeFigueiredo]

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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