I have this query in SQL used by my Haskell code:

let name = "A 20"
let qry_head_WDS = "select DNUM from WDS where DISC = ?" :: Query
putStrLn "before query WDS"
bd_rows_WDS <- query conn qry_head_WDS (Only (name::String))
putStrLn "after query WDS"

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

works well if there is no NULL values in a database, but if NULL value for field N°BD i got this exception at runtime:

before query WDS
*** Exception: UnexpectedNull {errSQLType = "String", errHaskellType =
"Text", errFieldName = "DNUM", errMessage = "unexpected null in table
WDS of database sidonie"}
*Main>

Can you help me to handle NULL values in the field N°BD

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damien.mattei@gmail.com wrote:

I have this query in SQL used by my Haskell code:

let name = "A 20"
let qry_head_WDS = "select DNUM from WDS where DISC = ?" :: Query
putStrLn "before query WDS"
bd_rows_WDS <- query conn qry_head_WDS (Only (name::String))
putStrLn "after query WDS"

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

works well if there is no NULL values in a database, but if NULL value for field N°BD i got this exception at runtime: [snip]

First, what kind of database are you using and which library are you using
to access it? This may make a difference to the solution.

I think you may be able to get the result you want by indicating to
Haskell's type inference that bd_rows_WDS should have type [Maybe Text], not [Only Text], since there actually are NULLs in your database and you need to
be able to handle them. Try replacing your last line with this:

forM_ bd_rows_WDS $ \row -> case row of
Nothing -> putStrLn "The value was NULL"
Just text -> putStrLn $ Text.unpack text

It may also be necessary to explicitly mention the type you expect for bd_rows_WDS:

bd_rows_WDS :: [Maybe Text] <-
query conn qry_head_WDS (Only (name::String))

Hope this helps,

Benjamin

--
Benjamin Esham
https://esham.io

--- SoupGate-Win32 v1.05
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On Tuesday, December 18, 2018 at 7:08:39 PM UTC+1, Benjamin Esham wrote:

damien.mattei@gmail.com wrote:

I have this query in SQL used by my Haskell code:

let name = "A 20"
let qry_head_WDS = "select DNUM from WDS where DISC = ?" :: Query
putStrLn "before query WDS"
bd_rows_WDS <- query conn qry_head_WDS (Only (name::String))
putStrLn "after query WDS"

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

works well if there is no NULL values in a database, but if NULL value for field N°BD i got this exception at runtime: [snip]

First, what kind of database are you using and which library are you using
to access it? This may make a difference to the solution.

I think you may be able to get the result you want by indicating to
Haskell's type inference that bd_rows_WDS should have type [Maybe Text], not [Only Text], since there actually are NULLs in your database and you need to be able to handle them. Try replacing your last line with this:

forM_ bd_rows_WDS $ \row -> case row of
Nothing -> putStrLn "The value was NULL"
Just text -> putStrLn $ Text.unpack text

It may also be necessary to explicitly mention the type you expect for bd_rows_WDS:

bd_rows_WDS :: [Maybe Text] <-
query conn qry_head_WDS (Only (name::String))

Hope this helps,

Benjamin

--
Benjamin Esham
https://esham.io

i think it's a good idea Benjamin but unfortunately i get an error i do not understand from compiler:

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

UpdateSidonie.hs:213:5: error:
Illegal type signature: ‘[Maybe Text]’
Type signatures are only allowed in patterns with ScopedTypeVariables
|
213 | bd_rows_WDS :: [Maybe Text] <- query conn qry_head_WDS (Only (name::String))
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Failed, no modules loaded.

Damien

--- SoupGate-Win32 v1.05
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damien.mattei@gmail.com wrote:

On Tuesday, December 18, 2018 at 7:08:39 PM UTC+1, Benjamin Esham wrote:

damien.mattei@gmail.com wrote:

I have this query in SQL used by my Haskell code:

let name = "A 20"
let qry_head_WDS = "select DNUM from WDS where DISC = ?" :: Query
putStrLn "before query WDS"
bd_rows_WDS <- query conn qry_head_WDS (Only (name::String))
putStrLn "after query WDS"

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

works well if there is no NULL values in a database, but if NULL value for >> > field N°BD i got this exception at runtime: [snip]

[snip]

It may also be necessary to explicitly mention the type you expect for
bd_rows_WDS:

bd_rows_WDS :: [Maybe Text] <-
query conn qry_head_WDS (Only (name::String))

i think it's a good idea Benjamin but unfortunately i get an error i do
not understand from compiler:

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

UpdateSidonie.hs:213:5: error:
Illegal type signature: ‘[Maybe Text]’
Type signatures are only allowed in patterns with ScopedTypeVariables
|
213 | bd_rows_WDS :: [Maybe Text] <- query conn qry_head_WDS (Only (name::String))
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Failed, no modules loaded.

Ah, sorry about that. I forgot that if you want to add a type annotation
like this you need to be using the ScopedTypeVariables extension. You can do that by adding the line

{-# LANGUAGE ScopedTypeVariables #-}

to your source file or by adding the line

default-extensions: ScopedTypeVariables

to your cabal file, under either the "executable" or "library" sections depending on which one of those you're creating.

I /also/ messed up the type I suggested that bd_rows_WDS should be. It's a
list of Only (Maybe Text), not a list of Maybe Text.

After playing around with a test database using postgres-simple, I came up
with this for your code:

let name = "A 20"
let qry_head_WDS = "select DNUM from WDS where DISC = ?" :: Query
putStrLn "before query WDS"
bd_rows_WDS :: [Only (Maybe Text)]
<- query conn qry_head_WDS (Only (name::String))
putStrLn "after query WDS"

forM_ bd_rows_WDS $ \(Only a) -> case a of
Just a' -> putStrLn $ Text.unpack a'
Nothing -> putStrLn "It's null"

However, in my testing, I found that the type annotation for bd_rows_WDS is actually not needed: Haskell is able to infer the correct type because the lambda that you pass to forM_ tells the compiler unambiguously that each element of bd_rows_WDS has type Only (Maybe Text).

So to fix your original problem, all you need to do is to change your lambda
as shown here; you don't need to add that ":: [Only (Maybe Text)]"
annotation or turn on ScopedTypeVariables. In some situations that may be necessary--or at least more convenient--but it turns out that it's not
needed here.

Hope this helps!

Benjamin

--
Benjamin Esham
https://esham.io

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