Haskell getting a string from list comprehension

Question

I'm going through Learn You A Haskell. I have a following function:

bmiTell :: (RealFloat a) => a -> a-> String
bmiTell weight height
  | bmi <= skinny ="underweight"
  | bmi <= normal = "ok"
  | bmi <= fat = "fat"
  | otherwise = "whale"
  where
    bmi = weight/height^2
    (skinny,normal,fat)=(18.5, 25.0,30.0)

which works perfectly fine. I'm now making a list comprehension, where a desired result is something like this:

[(68, "underweight"),(69,"ok"),(70,"ok")]

this is my ghci input:

[(x,y)| x <-[68..70], y <- bmiTell x 185]

and the output is

[(68.0,'u'),(68.0,'n'),(68.0,'d'),(68.0,'e'),(68.0,'r'),(68.0,'w'),(68.0,'e'),(68.0,'i'),(68.0,'g'),(68.0,'h'),(68.0,'t'),(69.0,'u'),(69.0,'n'),(69.0,'d'),(69.0,'e'),(69.0,'r'),(69.0,'w'),(69.0,'e'),(69.0,'i'),(69.0,'g'),(69.0,'h'),(69.0,'t'),(70.0,'u'),(70.0,'n'),(70.0,'d'),(70.0,'e'),(70.0,'r'),(70.0,'w'),(70.0,'e'),(70.0,'i'),(70.0,'g'),(70.0,'h'),(70.0,'t')]

I tried making it a (x,[y]), but I get the same result with Chars in ""s instead of single quotes

Indeed, thank you, may I have your answer please? so embarassed — justice
– justice, Commented Aug 14, 2016 at 11:23
Put bmiTell X 185 in a list [bmiTell X 185] and your's should work. Remember, strings are lists. The comprehension was pull}ng from the only list it found. Also, two generators in a comprehension are aXb or Cartesian — fp_mora
– fp_mora, Commented Mar 10, 2018 at 1:50

mb21 · Accepted Answer · 2016-08-14 11:33:32Z

3

You can simply use the map function:

map (\w -> (w, bmiTell w 185)) [68..70]

Or as @ErikR mentioned, using list comprehension:

[ (x, bmiTell x 185) | x <- [68..70] ]

edited Aug 14, 2016 at 11:33

answered Aug 14, 2016 at 11:24

mb21

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