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

[Damien Mattei]

 let noBD = fromOnly (Prelude.head bd_rows)

give me a solution, but i'm still searching a solution with map has in a
list there could be more than one result...

D

Le 06/12/2018 17:10, Francesco Ariis a écrit :
> On Thu, Dec 06, 2018 at 03:54:45PM +0100, Damien Mattei wrote:
>> if it was so easy!
> 
> Damien, you must heed the compiler!

heed?
> 
>> 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.
> 
> So you applied `fromOnly` to `bd_rows`. fromOnly has type
> 
>     fromOnly :: Only a -> a
> 
> Now GHC is complaining:
> 
>       • I was expecting ‘Only a’
>               but you gave me ‘[Only Text]’
> 
> So either call "head" on [Only Text] (unsafe!) or map over it.
> See if it works and if not, fire again here
> -F
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