database, list result, extraction and conversion
again un haskell beginner question ;-) ... i have this: [Only {fromOnly = "-04.3982"}] as result of a database query, i understand it's a list and i can get the first element: *Main> Data.List.head [Only {fromOnly = "-04.3982"}] Only {fromOnly = "-04.3982"} *Main> let s = Data.List.head [Only {fromOnly = "-04.3982"}] *Main> s Only {fromOnly = "-04.3982"} *Main> show s "Only {fromOnly = \"-04.3982\"}" but how can i get the String "-04.3982" ? and after converting it to a Float -- Damien.Mattei@unice.fr, Damien.Mattei@oca.eu, UNS / OCA / CNRS
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-Si...
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-Si...
_______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
-- Damien.Mattei@unice.fr, Damien.Mattei@oca.eu, UNS / OCA / CNRS
On Thu, Dec 06, 2018 at 03:54:45PM +0100, Damien Mattei wrote:
if it was so easy!
Damien, you must heed the compiler!
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
Thank you for reaching out to me.
Is this a remote position?
I live in Los Angeles and cannot relocate.
Thank You
Bill Kunyiha
On Thu, Dec 6, 2018 at 8:10 AM Francesco Ariis
On Thu, Dec 06, 2018 at 03:54:45PM +0100, Damien Mattei wrote:
if it was so easy!
Damien, you must heed the compiler!
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 _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
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 _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
On Fri, Dec 07, 2018 at 09:44:03AM +0100, Damien Mattei wrote:
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...
So put this in your code let noBD = map _ bd_rows `_` will tell the compiler "tell me which type goes there". Then you can choose the appropriate function!
yes thanks you i had swapped the arguments of map in my code, i lack sleeping those days this solution worked : let resLst = Prelude.map fromOnly bd_rows but not this: let noBD2 = Prelude.map _ bd_rows Prelude> :load UpdateSidonie [1 of 1] Compiling Main ( UpdateSidonie.hs, interpreted ) UpdateSidonie.hs:94:29: error: • Found hole: _ :: Only Text -> b Where: ‘b’ is a rigid type variable bound by the inferred type of noBD2 :: [b] at UpdateSidonie.hs:94:9-37 • In the first argument of ‘Prelude.map’, namely ‘_’ In the expression: Prelude.map _ bd_rows In an equation for ‘noBD2’: noBD2 = Prelude.map _ bd_rows • Relevant bindings include noBD2 :: [b] (bound at UpdateSidonie.hs:94:9) noBD :: Text (bound at UpdateSidonie.hs:93:9) resLst :: [Text] (bound at UpdateSidonie.hs:91:9) bd_rows :: [Only Text] (bound at UpdateSidonie.hs:86:5) qry_head :: Query (bound at UpdateSidonie.hs:79:9) qry :: [Char] (bound at UpdateSidonie.hs:77:9) (Some bindings suppressed; use -fmax-relevant-binds=N or -fno-max-relevant-binds) Valid substitutions include undefined :: forall (a :: TYPE r). GHC.Stack.Types.HasCallStack => a (imported from ‘Prelude’ at UpdateSidonie.hs:1:1 (and originally defined in ‘GHC.Err’)) | 94 | let noBD2 = Prelude.map _ bd_rows | ^ Failed, no modules loaded. it's annoying ,since i add 'import' ,i must Prelude.* basic functions: import Database.MySQL.Simple import Database.MySQL.Simple.QueryResults import Database.MySQL.Simple.Result import Control.Monad import Data.Text i must use Prelude.map, Prelude.head is there a solution to this problem? Le 07/12/2018 09:59, Francesco Ariis a écrit :
On Fri, Dec 07, 2018 at 09:44:03AM +0100, Damien Mattei wrote:
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...
So put this in your code
let noBD = map _ bd_rows
`_` will tell the compiler "tell me which type goes there". Then you can choose the appropriate function! _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
-- Damien.Mattei@unice.fr, Damien.Mattei@oca.eu, UNS / OCA / CNRS
participants (3)
-
Bill Kunyiha -
Damien Mattei -
Francesco Ariis