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?










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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, and rowLength do.

    – Daniel Wagner
    Jan 4 at 3:24


















0















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?










share|improve this question


















  • 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














0












0








0








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?










share|improve this question














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






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asked Jan 4 at 2:38









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 what splitEvery, rowLength, coords, table, allCoords, colLength, and rowLength do.

    – Daniel Wagner
    Jan 4 at 3:24














  • 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








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












1 Answer
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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) ]





share|improve this answer
























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    1 Answer
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    1 Answer
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    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) ]





    share|improve this answer




























      1














      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) ]





      share|improve this answer


























        1












        1








        1







        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) ]





        share|improve this answer













        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) ]






        share|improve this answer












        share|improve this answer



        share|improve this answer










        answered Jan 4 at 2:52









        talextalex

        11.9k11749




        11.9k11749
































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