How do I make this function add Nothing to the list?
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Is there any way to add Nothing to a list in a list comphrehension?
I have written the following function
toShape :: AltShape -> Shape
toShape ps = splitEvery (rowLength ps)
[ if (row `notElem` (map coords ps))
then (lookup row (table ps)):r
else Nothing
| row <- allCoords (colLength ps) (rowLength ps) ]
Where AltShape and Shape are defined as:
type AltShape = [Point]
data Point = P Colour (Int,Int) deriving (Eq,Show)
type Shape = [Row]
type Row = [Square]
type Square = Maybe Colour
data Colour = Black | Red | Green deriving (Eq,Show)
Basically the function is supposed to convert AltShapes to Shapes. I have been trying to accomplish this by creating a function which creates all possible coordinates based on the width and height of the shape, and then matching them to the actual points in the shape. If there is a match I want to add Just c to the list, where c is the colour, and if there isn't a match I want it to add Nothing to the list. But now I just end up with a list of Just c. How should I write it differently?
haskell
add a comment |
Is there any way to add Nothing to a list in a list comphrehension?
I have written the following function
toShape :: AltShape -> Shape
toShape ps = splitEvery (rowLength ps)
[ if (row `notElem` (map coords ps))
then (lookup row (table ps)):r
else Nothing
| row <- allCoords (colLength ps) (rowLength ps) ]
Where AltShape and Shape are defined as:
type AltShape = [Point]
data Point = P Colour (Int,Int) deriving (Eq,Show)
type Shape = [Row]
type Row = [Square]
type Square = Maybe Colour
data Colour = Black | Red | Green deriving (Eq,Show)
Basically the function is supposed to convert AltShapes to Shapes. I have been trying to accomplish this by creating a function which creates all possible coordinates based on the width and height of the shape, and then matching them to the actual points in the shape. If there is a match I want to add Just c to the list, where c is the colour, and if there isn't a match I want it to add Nothing to the list. But now I just end up with a list of Just c. How should I write it differently?
haskell
4
Um. Can you include enough code that we can reproduce your problem, please? Lots of stuff about this snippet looks off, but it's tough to tell for sure without knowing whatsplitEvery
,rowLength
,coords
,table
,allCoords
,colLength
, androwLength
do.
– Daniel Wagner
Jan 4 at 3:24
add a comment |
Is there any way to add Nothing to a list in a list comphrehension?
I have written the following function
toShape :: AltShape -> Shape
toShape ps = splitEvery (rowLength ps)
[ if (row `notElem` (map coords ps))
then (lookup row (table ps)):r
else Nothing
| row <- allCoords (colLength ps) (rowLength ps) ]
Where AltShape and Shape are defined as:
type AltShape = [Point]
data Point = P Colour (Int,Int) deriving (Eq,Show)
type Shape = [Row]
type Row = [Square]
type Square = Maybe Colour
data Colour = Black | Red | Green deriving (Eq,Show)
Basically the function is supposed to convert AltShapes to Shapes. I have been trying to accomplish this by creating a function which creates all possible coordinates based on the width and height of the shape, and then matching them to the actual points in the shape. If there is a match I want to add Just c to the list, where c is the colour, and if there isn't a match I want it to add Nothing to the list. But now I just end up with a list of Just c. How should I write it differently?
haskell
Is there any way to add Nothing to a list in a list comphrehension?
I have written the following function
toShape :: AltShape -> Shape
toShape ps = splitEvery (rowLength ps)
[ if (row `notElem` (map coords ps))
then (lookup row (table ps)):r
else Nothing
| row <- allCoords (colLength ps) (rowLength ps) ]
Where AltShape and Shape are defined as:
type AltShape = [Point]
data Point = P Colour (Int,Int) deriving (Eq,Show)
type Shape = [Row]
type Row = [Square]
type Square = Maybe Colour
data Colour = Black | Red | Green deriving (Eq,Show)
Basically the function is supposed to convert AltShapes to Shapes. I have been trying to accomplish this by creating a function which creates all possible coordinates based on the width and height of the shape, and then matching them to the actual points in the shape. If there is a match I want to add Just c to the list, where c is the colour, and if there isn't a match I want it to add Nothing to the list. But now I just end up with a list of Just c. How should I write it differently?
haskell
haskell
asked Jan 4 at 2:38
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FrausFraus
314
314
4
Um. Can you include enough code that we can reproduce your problem, please? Lots of stuff about this snippet looks off, but it's tough to tell for sure without knowing whatsplitEvery
,rowLength
,coords
,table
,allCoords
,colLength
, androwLength
do.
– Daniel Wagner
Jan 4 at 3:24
add a comment |
4
Um. Can you include enough code that we can reproduce your problem, please? Lots of stuff about this snippet looks off, but it's tough to tell for sure without knowing whatsplitEvery
,rowLength
,coords
,table
,allCoords
,colLength
, androwLength
do.
– Daniel Wagner
Jan 4 at 3:24
4
4
Um. Can you include enough code that we can reproduce your problem, please? Lots of stuff about this snippet looks off, but it's tough to tell for sure without knowing what
splitEvery
, rowLength
, coords
, table
, allCoords
, colLength
, and rowLength
do.– Daniel Wagner
Jan 4 at 3:24
Um. Can you include enough code that we can reproduce your problem, please? Lots of stuff about this snippet looks off, but it's tough to tell for sure without knowing what
splitEvery
, rowLength
, coords
, table
, allCoords
, colLength
, and rowLength
do.– Daniel Wagner
Jan 4 at 3:24
add a comment |
1 Answer
1
active
oldest
votes
Use guards
toShape :: AltShape -> Shape
toShape ps = splitEvery (rowLength ps)
[ (lookup row (table ps)):r
| row <- allCoords (colLength ps) (rowLength ps)
, row `notElem` (map coords ps) ]
add a comment |
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1 Answer
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oldest
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1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
Use guards
toShape :: AltShape -> Shape
toShape ps = splitEvery (rowLength ps)
[ (lookup row (table ps)):r
| row <- allCoords (colLength ps) (rowLength ps)
, row `notElem` (map coords ps) ]
add a comment |
Use guards
toShape :: AltShape -> Shape
toShape ps = splitEvery (rowLength ps)
[ (lookup row (table ps)):r
| row <- allCoords (colLength ps) (rowLength ps)
, row `notElem` (map coords ps) ]
add a comment |
Use guards
toShape :: AltShape -> Shape
toShape ps = splitEvery (rowLength ps)
[ (lookup row (table ps)):r
| row <- allCoords (colLength ps) (rowLength ps)
, row `notElem` (map coords ps) ]
Use guards
toShape :: AltShape -> Shape
toShape ps = splitEvery (rowLength ps)
[ (lookup row (table ps)):r
| row <- allCoords (colLength ps) (rowLength ps)
, row `notElem` (map coords ps) ]
answered Jan 4 at 2:52
talextalex
11.9k11749
11.9k11749
add a comment |
add a comment |
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4
Um. Can you include enough code that we can reproduce your problem, please? Lots of stuff about this snippet looks off, but it's tough to tell for sure without knowing what
splitEvery
,rowLength
,coords
,table
,allCoords
,colLength
, androwLength
do.– Daniel Wagner
Jan 4 at 3:24