Mixing
converting Maybe type1 in type1 after filtering
0
i had filtered a [Maybe Text] type to remove Nothing from the list and now i want to put the result in a [Text] type but the compiler complains about the incompatible type :
(bd_rows_WDS :: [Only (Maybe Text)]) <- query conn qry_head_WDS (Only (name::String))
-- remove the records having N°BD NULL
let fltWDS :: [Only Text] = Prelude.filter ((Only a) ->
case a of
Nothing -> False
Just a -> True)
bd_rows_WDS
Prelude> :load UpdateSidonie
[1 of 1] Compiling Main ( UpdateSidonie.hs, interpreted )
UpdateSidonie.hs:282:33: error:
• Couldn't match type ‘Maybe Text’ with ‘Text’
Expected type: [Only Text]
Actual type: [Only (Maybe Text)]
• In the expression:
Prelude.filter
( (Only a)
-> case a of
Nothing -> False
Just a -> True)
bd_rows_WDS
In a pattern binding:
fltWDS :: [Only Text]
= Prelude.filter
( (Only a)
-> case a of
Nothing -> False
Just a -> True)
bd_rows_WDS
In the expression:
do conn <- connect
defaultConnectInfo
{connectHost = "moita", connectUser = "mattei",
connectPassword = "sidonie2", connectDatabase = "sidonie"}
(rows :: [(Text, Double)]) <- query_
conn
"SELECT Nom,distance FROM AngularDistance WHERE distance > 0.000278"
(names :: [Only Text]) <- query_
conn
"SELECT Nom FROM AngularDistance WHERE distance > 0.000278"
let resLstNames = Prelude.map fromOnly names
....
|
282 | let fltWDS :: [Only Text] = Prelude.filter ((Only a) ->
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^...
Failed, no modules loaded.
how can id do the type conversion,now i'm sure there is no more Nothing values?
haskell
asked Jan 2 at 8:14
Damien MatteiDamien Mattei
164
add a comment |
0
i had filtered a [Maybe Text] type to remove Nothing from the list and now i want to put the result in a [Text] type but the compiler complains about the incompatible type :
(bd_rows_WDS :: [Only (Maybe Text)]) <- query conn qry_head_WDS (Only (name::String))
-- remove the records having N°BD NULL
let fltWDS :: [Only Text] = Prelude.filter ((Only a) ->
case a of
Nothing -> False
Just a -> True)
bd_rows_WDS
Prelude> :load UpdateSidonie
[1 of 1] Compiling Main ( UpdateSidonie.hs, interpreted )
UpdateSidonie.hs:282:33: error:
• Couldn't match type ‘Maybe Text’ with ‘Text’
Expected type: [Only Text]
Actual type: [Only (Maybe Text)]
• In the expression:
Prelude.filter
( (Only a)
-> case a of
Nothing -> False
Just a -> True)
bd_rows_WDS
In a pattern binding:
fltWDS :: [Only Text]
= Prelude.filter
( (Only a)
-> case a of
Nothing -> False
Just a -> True)
bd_rows_WDS
In the expression:
do conn <- connect
defaultConnectInfo
{connectHost = "moita", connectUser = "mattei",
connectPassword = "sidonie2", connectDatabase = "sidonie"}
(rows :: [(Text, Double)]) <- query_
conn
"SELECT Nom,distance FROM AngularDistance WHERE distance > 0.000278"
(names :: [Only Text]) <- query_
conn
"SELECT Nom FROM AngularDistance WHERE distance > 0.000278"
let resLstNames = Prelude.map fromOnly names
....
|
282 | let fltWDS :: [Only Text] = Prelude.filter ((Only a) ->
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^...
Failed, no modules loaded.
how can id do the type conversion,now i'm sure there is no more Nothing values?
haskell
asked Jan 2 at 8:14
Damien MatteiDamien Mattei
164
add a comment |
0
0
0
i had filtered a [Maybe Text] type to remove Nothing from the list and now i want to put the result in a [Text] type but the compiler complains about the incompatible type :
(bd_rows_WDS :: [Only (Maybe Text)]) <- query conn qry_head_WDS (Only (name::String))
-- remove the records having N°BD NULL
let fltWDS :: [Only Text] = Prelude.filter ((Only a) ->
case a of
Nothing -> False
Just a -> True)
bd_rows_WDS
Prelude> :load UpdateSidonie
[1 of 1] Compiling Main ( UpdateSidonie.hs, interpreted )
UpdateSidonie.hs:282:33: error:
• Couldn't match type ‘Maybe Text’ with ‘Text’
Expected type: [Only Text]
Actual type: [Only (Maybe Text)]
• In the expression:
Prelude.filter
( (Only a)
-> case a of
Nothing -> False
Just a -> True)
bd_rows_WDS
In a pattern binding:
fltWDS :: [Only Text]
= Prelude.filter
( (Only a)
-> case a of
Nothing -> False
Just a -> True)
bd_rows_WDS
In the expression:
do conn <- connect
defaultConnectInfo
{connectHost = "moita", connectUser = "mattei",
connectPassword = "sidonie2", connectDatabase = "sidonie"}
(rows :: [(Text, Double)]) <- query_
conn
"SELECT Nom,distance FROM AngularDistance WHERE distance > 0.000278"
(names :: [Only Text]) <- query_
conn
"SELECT Nom FROM AngularDistance WHERE distance > 0.000278"
let resLstNames = Prelude.map fromOnly names
....
|
282 | let fltWDS :: [Only Text] = Prelude.filter ((Only a) ->
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^...
Failed, no modules loaded.
how can id do the type conversion,now i'm sure there is no more Nothing values?
haskell
asked Jan 2 at 8:14
Damien MatteiDamien Mattei
164
i had filtered a [Maybe Text] type to remove Nothing from the list and now i want to put the result in a [Text] type but the compiler complains about the incompatible type :
(bd_rows_WDS :: [Only (Maybe Text)]) <- query conn qry_head_WDS (Only (name::String))
-- remove the records having N°BD NULL
let fltWDS :: [Only Text] = Prelude.filter ((Only a) ->
case a of
Nothing -> False
Just a -> True)
bd_rows_WDS
Prelude> :load UpdateSidonie
[1 of 1] Compiling Main ( UpdateSidonie.hs, interpreted )
UpdateSidonie.hs:282:33: error:
• Couldn't match type ‘Maybe Text’ with ‘Text’
Expected type: [Only Text]
Actual type: [Only (Maybe Text)]
• In the expression:
Prelude.filter
( (Only a)
-> case a of
Nothing -> False
Just a -> True)
bd_rows_WDS
In a pattern binding:
fltWDS :: [Only Text]
= Prelude.filter
( (Only a)
-> case a of
Nothing -> False
Just a -> True)
bd_rows_WDS
In the expression:
do conn <- connect
defaultConnectInfo
{connectHost = "moita", connectUser = "mattei",
connectPassword = "sidonie2", connectDatabase = "sidonie"}
(rows :: [(Text, Double)]) <- query_
conn
"SELECT Nom,distance FROM AngularDistance WHERE distance > 0.000278"
(names :: [Only Text]) <- query_
conn
"SELECT Nom FROM AngularDistance WHERE distance > 0.000278"
let resLstNames = Prelude.map fromOnly names
....
|
282 | let fltWDS :: [Only Text] = Prelude.filter ((Only a) ->
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^...
Failed, no modules loaded.
how can id do the type conversion,now i'm sure there is no more Nothing values?
haskell
haskell
asked Jan 2 at 8:14
Damien MatteiDamien Mattei
164
asked Jan 2 at 8:14
Damien MatteiDamien Mattei
164
asked Jan 2 at 8:14
Damien MatteiDamien Mattei
164
asked Jan 2 at 8:14
Damien MatteiDamien Mattei
164
asked Jan 2 at 8:14
Damien MatteiDamien Mattei
164
164
add a comment |
add a comment |
1 Answer
1
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oldest
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6
You can use catMaybes from Data.Maybe.
catMaybes :: [Maybe a] -> [a]
PS: It also filter out Nothing so you do't need to do it yourself.
answered Jan 2 at 8:16
talextalex
11.5k11648
i try your answer in my code inserting this let fltWDS2 = catMaybes bd_rows_WDS but get the same error : Couldn't match type ‘Only (Maybe Text)’ with ‘Maybe a’
– Damien Mattei
Jan 2 at 8:37
Are you sure error exactly same? I notice that your type not[Maybe Text], but[Only (Maybe Text)]what isOnly?
– talex
Jan 2 at 8:58
bd_rows_WDS is with Only, fltWDS2 has no type specified
– Damien Mattei
Jan 2 at 13:44
What isOnly? I'm unable to find definition in hoogle.
– talex
Jan 3 at 2:32
i'm new to haskell i had to use Only for 1-tuple type with database when there is Only one argument or Only one column result i suppose when i used examples about database query, here is the package: hackage.haskell.org/package/Only
– Damien Mattei
Jan 4 at 7:24
add a comment |
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1 Answer
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1 Answer
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active
oldest
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oldest
votes
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oldest
votes
6
You can use catMaybes from Data.Maybe.
catMaybes :: [Maybe a] -> [a]
PS: It also filter out Nothing so you do't need to do it yourself.
answered Jan 2 at 8:16
talextalex
11.5k11648
i try your answer in my code inserting this let fltWDS2 = catMaybes bd_rows_WDS but get the same error : Couldn't match type ‘Only (Maybe Text)’ with ‘Maybe a’
– Damien Mattei
Jan 2 at 8:37
Are you sure error exactly same? I notice that your type not[Maybe Text], but[Only (Maybe Text)]what isOnly?
– talex
Jan 2 at 8:58
bd_rows_WDS is with Only, fltWDS2 has no type specified
– Damien Mattei
Jan 2 at 13:44
What isOnly? I'm unable to find definition in hoogle.
– talex
Jan 3 at 2:32
i'm new to haskell i had to use Only for 1-tuple type with database when there is Only one argument or Only one column result i suppose when i used examples about database query, here is the package: hackage.haskell.org/package/Only
– Damien Mattei
Jan 4 at 7:24
add a comment |
6
You can use catMaybes from Data.Maybe.
catMaybes :: [Maybe a] -> [a]
PS: It also filter out Nothing so you do't need to do it yourself.
answered Jan 2 at 8:16
talextalex
11.5k11648
i try your answer in my code inserting this let fltWDS2 = catMaybes bd_rows_WDS but get the same error : Couldn't match type ‘Only (Maybe Text)’ with ‘Maybe a’
– Damien Mattei
Jan 2 at 8:37
Are you sure error exactly same? I notice that your type not[Maybe Text], but[Only (Maybe Text)]what isOnly?
– talex
Jan 2 at 8:58
bd_rows_WDS is with Only, fltWDS2 has no type specified
– Damien Mattei
Jan 2 at 13:44
What isOnly? I'm unable to find definition in hoogle.
– talex
Jan 3 at 2:32
i'm new to haskell i had to use Only for 1-tuple type with database when there is Only one argument or Only one column result i suppose when i used examples about database query, here is the package: hackage.haskell.org/package/Only
– Damien Mattei
Jan 4 at 7:24
add a comment |
6
6
6
You can use catMaybes from Data.Maybe.
catMaybes :: [Maybe a] -> [a]
PS: It also filter out Nothing so you do't need to do it yourself.
answered Jan 2 at 8:16
talextalex
11.5k11648
You can use catMaybes from Data.Maybe.
catMaybes :: [Maybe a] -> [a]
PS: It also filter out Nothing so you do't need to do it yourself.
answered Jan 2 at 8:16
talextalex
11.5k11648
answered Jan 2 at 8:16
talextalex
11.5k11648
answered Jan 2 at 8:16
talextalex
11.5k11648
answered Jan 2 at 8:16
talextalex
11.5k11648
11.5k11648
i try your answer in my code inserting this let fltWDS2 = catMaybes bd_rows_WDS but get the same error : Couldn't match type ‘Only (Maybe Text)’ with ‘Maybe a’
– Damien Mattei
Jan 2 at 8:37
Are you sure error exactly same? I notice that your type not[Maybe Text], but[Only (Maybe Text)]what isOnly?
– talex
Jan 2 at 8:58
bd_rows_WDS is with Only, fltWDS2 has no type specified
– Damien Mattei
Jan 2 at 13:44
What isOnly? I'm unable to find definition in hoogle.
– talex
Jan 3 at 2:32
i'm new to haskell i had to use Only for 1-tuple type with database when there is Only one argument or Only one column result i suppose when i used examples about database query, here is the package: hackage.haskell.org/package/Only
– Damien Mattei
Jan 4 at 7:24
add a comment |
i try your answer in my code inserting this let fltWDS2 = catMaybes bd_rows_WDS but get the same error : Couldn't match type ‘Only (Maybe Text)’ with ‘Maybe a’
– Damien Mattei
Jan 2 at 8:37
Are you sure error exactly same? I notice that your type not[Maybe Text], but[Only (Maybe Text)]what isOnly?
– talex
Jan 2 at 8:58
bd_rows_WDS is with Only, fltWDS2 has no type specified
– Damien Mattei
Jan 2 at 13:44
What isOnly? I'm unable to find definition in hoogle.
– talex
Jan 3 at 2:32
i'm new to haskell i had to use Only for 1-tuple type with database when there is Only one argument or Only one column result i suppose when i used examples about database query, here is the package: hackage.haskell.org/package/Only
– Damien Mattei
Jan 4 at 7:24
i try your answer in my code inserting this let fltWDS2 = catMaybes bd_rows_WDS but get the same error : Couldn't match type ‘Only (Maybe Text)’ with ‘Maybe a’
– Damien Mattei
Jan 2 at 8:37
i try your answer in my code inserting this let fltWDS2 = catMaybes bd_rows_WDS but get the same error : Couldn't match type ‘Only (Maybe Text)’ with ‘Maybe a’
– Damien Mattei
Jan 2 at 8:37
Are you sure error exactly same? I notice that your type not
[Maybe Text], but [Only (Maybe Text)] what is Only?– talex
Jan 2 at 8:58
Are you sure error exactly same? I notice that your type not
[Maybe Text], but [Only (Maybe Text)] what is Only?– talex
Jan 2 at 8:58
bd_rows_WDS is with Only, fltWDS2 has no type specified
– Damien Mattei
Jan 2 at 13:44
bd_rows_WDS is with Only, fltWDS2 has no type specified
– Damien Mattei
Jan 2 at 13:44
What is
Only? I'm unable to find definition in hoogle.– talex
Jan 3 at 2:32
What is
Only? I'm unable to find definition in hoogle.– talex
Jan 3 at 2:32
i'm new to haskell i had to use Only for 1-tuple type with database when there is Only one argument or Only one column result i suppose when i used examples about database query, here is the package: hackage.haskell.org/package/Only
– Damien Mattei
Jan 4 at 7:24
i'm new to haskell i had to use Only for 1-tuple type with database when there is Only one argument or Only one column result i suppose when i used examples about database query, here is the package: hackage.haskell.org/package/Only
– Damien Mattei
Jan 4 at 7:24
add a comment |
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