[Haskell-beginners] (SPAM 3)Re: database, list result, extraction and conversion

[Damien Mattei]

if it was so easy!

GHCi, version 8.4.3: http://www.haskell.org/ghc/  :? for help
Prelude> fromOnly  [Only {fromOnly = "-04.3982"}]

<interactive>:1:12: error: Not in scope: data constructor ‘Only’


and in a program:

bd_rows <- query conn qry_head (Only (name::String))

    putStrLn $ show bd_rows
    putStrLn $ show name

    let resLst = []

    let noBD = fromOnly bd_rows



    forM_ bd_rows $ \(Only a) ->
      putStrLn $  Data.Text.unpack a


Prelude> :load UpdateSidonie
[1 of 1] Compiling Main             ( UpdateSidonie.hs, interpreted )

UpdateSidonie.hs:93:25: error:
    • Couldn't match expected type ‘Only a’
                  with actual type ‘[Only Text]’
    • In the first argument of ‘fromOnly’, namely ‘bd_rows’
      In the expression: fromOnly bd_rows
      In an equation for ‘noBD’: noBD = fromOnly bd_rows
    • Relevant bindings include
        noBD :: a (bound at UpdateSidonie.hs:93:9)
   |
93 |     let noBD = fromOnly bd_rows
   |                         ^^^^^^^
Failed, no modules loaded.




Le 06/12/2018 14:07, Francesco Ariis a écrit :
> On Thu, Dec 06, 2018 at 12:16:08PM +0100, Damien Mattei wrote:
>> *Main> show s
>> "Only {fromOnly = \"-04.3982\"}"
>>
>> but how can i get the String "-04.3982" ? and after converting it to a
>> Float?
> 
> I suspect that "fromOnly" accessor will do!
> 
> http://hackage.haskell.org/package/mysql-simple-0.4.5/docs/Database-MySQL-Simple.html#t:Only
> 
> 
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-- 
Damien.Mattei at unice.fr, Damien.Mattei at oca.eu, UNS / OCA / CNRS