Hi I am a newbie in Haskell. I am trying to do a simple task.
test :: (RealFloat a) => a -> a -> [a]
test xs ys= [ w : h: [] | w <- xs, h <- ys]
I am getting an error here. (with out a doubt) In this task, I am simply trying to bind two 开发者_如何学Pythonlists (ex: test [12.12] [14.14]) and hopefully return a new combined list (ex: [12.12,14.14])
thanks for your help
Your signature is wrong. Try:
test xs ys = ...
then in ghci:
> :t test
test :: [t] -> [t] -> [[t]]
You need two arguments, both are lists, not two arguments of single elements.
Drakosha is correct. List concatenation already has an operator in Haskell.
test :: (RealFloat a) => [a] -> [a] -> [a]
test xs ys= xs ++ ys
You probably don't want to use a list comprehension here, unless you want to extract every element in your first and second list and do something with them. For example, a Cartesian Product:
list1 = [1.0,1.1,1.2] :: [Double]
list2 = [2.0,2.1,2.2] :: [Double]
testComps xs ys = [(x,y) | x <- xs, y <- ys]
Or addition:
testComps2 xs ys = [ x + y | x <- xs, y <- ys]
Or even creating lists:
testComps3 xs ys = [x : y : [] | x <- xs, y <- ys]
In GHCi, this will yield the following:
*Main> testComps list1 list2
[(1.0,2.0),(1.0,2.1),(1.0,2.2),(1.1,2.0),(1.1,2.1),(1.1,2.2),(1.2,2.0),(1.2,2.1)
,(1.2,2.2)]
*Main> testComps2 list1 list2
[3.0,3.1,3.2,3.1,3.2,3.3000000000000003,3.2,3.3,3.4000000000000004]
*Main> testComps3 list1 list2
[[1.0,2.0],[1.0,2.1],[1.0,2.2],[1.1,2.0],[1.1,2.1],[1.1,2.2],[1.2,2.0],[1.2,2.1]
,[1.2,2.2]]
The weird results in testComps2 is, of course, normal cruft when you're dealing with floating-point numbers. In the real world you'd compensate for this by rounding.
Another problem you'll run into is the difference between (++) and (:). Simply put, (:) tacks individual items onto a list, whereas (++) concatenates two lists.
You need list concatenation:
[12.12] ++ [14.14]
=> [12.12,14.14]
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