Haskell if else list comprehension - haskell

Question

I'm trying to print out a board in the right way, like this:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
1. . . . . . . . . . . . . . . . . .
2. . . . . . . . . . . . . . . . . .
3. . . . . . . . . . . . . . . . . .
4. . . . . . . . . . . . . . . . . .
5. . . . . . . . . . . . . . . . . .
6. . . . . . . . . . . . . . . . . .
7. . . . . . . . . . . . . . . . . .
8. . . . . . . . . . . . . . . . . .
9. . . . . . . . . . . . . . . . . .
10. . . . . . . . . . . . . . . . . .
11. . . . . . . . . . . . . . . . . .
12. . . . . . . . . . . . . . . . . .
13. . . . . . . . . . . . . . . . . .
14. . . . . . . . . . . . . . . . . .
15. . . . . . . . . . . . . . . . . .
16. . . . . . . . . . . . . . . . . .
17. . . . . . . . . . . . . . . . . .
18. . . . . . . . . . . . . . . . . .
I have the list comprehension here
((concat [(""++show i)++" " | i <- [1..n], i<10])++"\n")
I can get the 9 first numbers correctly, but the problem comes when i would like to put an else statement in my comprehension. I can't seem to know how to do that. So to make my self extra clear, i would like to do the same thing with the double digits but the only difference will be that i want two spaces instead of three between each double digit.

Answer 1

The left hand of the list comprehension accepts any Haskell expression, so we can write the if-then-else in the left hand of the list comprehension and omit the i < 10 condition on the right hand:
concat [(""++show i)++if i < 10 then " " else " " | i &lt- [1..n]]++"\n"
For n = 15, this then produces:
Prelude> concat [(""++show i)++if i < 10 then " " else " " | i <- [1..n]]++"\n"
"1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 \n"
We can also remove the ""++ part on the left side of the list comprehension, since this is bascially a no operation:
concat [show i ++ if i < 10 then " " else " " | i &lt- [1..n]] ++ "\n"
The above is however not very elegant: if i = 123, then we still will run into trouble. We can calculate the length of the show, calculate 4-l (with l the length), and add this as extra spacing. For instance:
concat [s ++ replicate (4-length s) ' ' | i &lt- [1..n], let s = show i] ++ "\n"
There are also some more dedicated formatting and joining functions, but since the exercise is probably to get familier with lists, I think this is out of scope here.
Like #4castle says, we can use for instance printf:
import Text.Printf(printf)
concatMap (printf "%-4d") [1..n] ++ "\n"