On Fri, Dec 14, 2018 at 11:22:38AM +0000, Tom Ellis wrote:

On Fri, Dec 14, 2018 at 11:49:45AM +0100, Damien Mattei wrote:

...

another question with Haskell, again with my database but that is again related to Monads:

Damien, it's very hard to read your posts because they consist of snippets of code interspersed with commentary. You'll probably get much better responses if you post single, very small, well-formatted examples of sample code.

Anyway, the ultimate answer to your query is "use do-notation". Your getBS_WDS would be clearer written something like:

Ah, I see now why you were determined to use `fmap`. Seph recommended it to you I would avoid using do-notation at all until you’re a bit more comfortable with how monadic computations work, and how Haskell handles IO. To someone used to imperative programming, it can be more confusing that helpful, as it allows you to write something that looks and feels a lot like imperative code but differs from it in crucial ways. Get comfortable with using the monadic operators >> and >>= directly, and only then switch back to do-notation. Or don’t—personally I prefer not to use do-notation at all. -- Seph Shewell Brockway https://mail.haskell.org/pipermail/haskell-cafe/2018-December/130367.html I have to say I have completely the opposite position to Seph. If you try to write code in IO as a beginner without using do-notation you are in for a world of pain. Damien, I would suggest you write (very) small practice programs using both Seph's advice and mine and see which style you prefer and which helps you learn Haskell quicker. Tom

ok Elis i tried again, first ,as i am familiar with λ expressions i can use both do syntactic sugar or the other form with λ. so itried you code,surpringly it compiles but when i launch it the function can not print somethin on screen, i assume it is because function are pure in haskell, nor with putStrln or Debug.trace: getBD_WDS :: Connection -> String -> IO Float getBD_WDS conn name = do let qry_head = "select DNUM from WDS where DISC = ?" :: Query bd_rows <- query conn qry_head (Only name) let lg = Prelude.length bd_rows -- putStrLn (if lg > 1 then "some" else "one or zero") let tst = trace (if lg > 1 then "some" else "one or zero") (if lg > 1 then "some" else "one or zero") -- let noBDtxt :: Text let noBDtxt = fromOnly (Prelude.head bd_rows) -- noBDstr :: String let noBDstr = Text.unpack noBDtxt -- noBDfp :: Float let noBDfp = read noBDstr return noBDfp in fact due to lazy evaluation i'm not sure it compiles all the code because a more simple example give compilation errors: getLstBD_WDS :: Connection -> String -> IO Int getLstBD_WDS conn name = do let qry_head = "select DNUM from WDS where DISC = ?" :: Query bd_rows <- query conn qry_head (Only name) let lg = Prelude.length bd_rows return lg Prelude> :load UpdateSidonie [1 of 1] Compiling Main ( UpdateSidonie.hs, interpreted ) UpdateSidonie.hs:123:14: error: • Ambiguous type variable ‘r0’ arising from a use of ‘query’ prevents the constraint ‘(QueryResults r0)’ from being solved. Probable fix: use a type annotation to specify what ‘r0’ should be. These potential instances exist: instance Result a => QueryResults (Only a) -- Defined in ‘Database.MySQL.Simple.QueryResults’ instance (Result a, Result b) => QueryResults (a, b) -- Defined in ‘Database.MySQL.Simple.QueryResults’ instance (Result a, Result b, Result c) => QueryResults (a, b, c) -- Defined in ‘Database.MySQL.Simple.QueryResults’ ...plus 21 others (use -fprint-potential-instances to see them all) • In a stmt of a 'do' block: bd_rows <- query conn qry_head (Only name) In the expression: do let qry_head = ... bd_rows <- query conn qry_head (Only name) let lg = Prelude.length bd_rows return lg In an equation for ‘getLstBD_WDS’: getLstBD_WDS conn name = do let qry_head = ... bd_rows <- query conn qry_head (Only name) let lg = ... .... | 123 | bd_rows <- query conn qry_head (Only name) | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Failed, no modules loaded. Le 14/12/2018 12:45, Tom Ellis a écrit :

On Fri, Dec 14, 2018 at 11:22:38AM +0000, Tom Ellis wrote:

...

On Fri, Dec 14, 2018 at 11:49:45AM +0100, Damien Mattei wrote:

...

another question with Haskell, again with my database but that is again related to Monads:

Damien, it's very hard to read your posts because they consist of snippets of code interspersed with commentary. You'll probably get much better responses if you post single, very small, well-formatted examples of sample code.

Anyway, the ultimate answer to your query is "use do-notation". Your getBS_WDS would be clearer written something like:

And ultimately I think you wanted to do something with lg and tst:

``` getBD_WDS :: Connection -> String -> IO Float getBD_WDS conn name = do let qry_head = "select DNUM from WDS where DISC = ?" :: Query

bd_rows <- query conn qry_head (Only name)

let lg = Prelude.length bd_rows

putStrLn (if lg > 1 then "some" else "one or zero")

let noBDtxt :: Text noBDtxt = fromOnly (Prelude.head bd_rows)

noBDstr :: String noBDstr = Text.unpack noBDtxt

noBDfp :: Float noBDfp = read noBDstr

return noBDfp ```

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yes i must admit but things are more and more weirds.... non i do not know why the first of those lines is OK and the other not for compilation: bd_rows <- query conn qry_head (Only (name::String)) bd_rows_WDS <- query conn qry_head_WDS (Only (name::String)) note:if i swap the order it always the line with qry_head_WDS that fails: *Main> :load UpdateSidonie [1 of 1] Compiling Main ( UpdateSidonie.hs, interpreted ) UpdateSidonie.hs:202:20: error: • Ambiguous type variable ‘r0’ arising from a use of ‘query’ prevents the constraint ‘(QueryResults r0)’ from being solved. Probable fix: use a type annotation to specify what ‘r0’ should be. These potential instances exist: instance Result a => QueryResults (Only a) -- Defined in ‘Database.MySQL.Simple.QueryResults’ instance (Result a, Result b) => QueryResults (a, b) -- Defined in ‘Database.MySQL.Simple.QueryResults’ instance (Result a, Result b, Result c) => QueryResults (a, b, c) -- Defined in ‘Database.MySQL.Simple.QueryResults’ ...plus 21 others (use -fprint-potential-instances to see them all) • In a stmt of a 'do' block: bd_rows_WDS <- query conn qry_head_WDS (Only (name :: String)) In the expression: do conn <- connect defaultConnectInfo {connectHost = "moita", connectUser = "mattei", connectPassword = "sidonie2", connectDatabase = "sidonie"} rows <- query_ conn "SELECT Nom,distance FROM AngularDistance WHERE distance > 0.000278" let lg = Prelude.length rows let tst = ... .... In an equation for ‘main’: main = do conn <- connect defaultConnectInfo {connectHost = "moita", connectUser = "mattei", connectPassword = "sidonie2", connectDatabase = "sidonie"} rows <- query_ conn "SELECT Nom,distance FROM AngularDistance WHERE distance > 0.000278" let lg = ... .... | 202 | bd_rows_WDS <- query conn qry_head_WDS (Only (name::String)) | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Failed, no modules loaded. for the curious i attach the whole code (UpdateSidonie.hs), i'm really tired of Haskell.... i hope with the file it will be more easy to help me... Le 14/12/2018 16:14, Brandon Allbery a écrit :

On Fri, Dec 14, 2018 at 9:36 AM Damien Mattei mailto:mattei@oca.eu> wrote:

in fact due to lazy evaluation i'm not sure it compiles all the code because a more simple example give compilation errors:

Er? Lazy evaluation has nothing to do with what gets compiled; the only way the compiler doesn't "compile all the code" is if you are using the CPP extension and #ifdef etc. to exclude some.

-- brandon s allbery kf8nh allbery.b@gmail.com mailto:allbery.b@gmail.com

-- Damien.Mattei@unice.fr, Damien.Mattei@oca.eu, UNS / OCA / CNRS