[Haskell-beginners] help with IO guards

[Sylvain Henry]

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 at 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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