CODEWITHSUNDEEP
haskell
Narwhal
Narwhal

Reputation: 335

Am I allowed to use "in" and "where" within a haskell definition?

I'm in the process of learning haskell, so I tried to make a the following function to see if I could do it.

This is what I wrote

projectileY :: (Num a, Fractional a) => a -> a -> a -> a -> a
projectileY gravity time vInit height = let equation gr ti veli hei = -0.5*g*(t^2)+vi*t+h
                                        in if (equation g t vi h < 0) then 0 else (equation g t vi h)
                                            where
                                                g = gravity
                                                t = time
                                                vi = vInit
                                                h = height

main :: IO()
main = do
  print (projectileY 9.8 0.1 2.0 100)

This was the error

parse error (possibly incorrect indentation or mismatched brackets)

So I tried messing with the indentation for a while, with no success. Am I allowed to use "in" and "where" like this?

Upvotes: 0

Views: 115

Answers (2)

ach
ach

Reputation: 2373

In addition to what others wrote about indents, it hardly makes sense to define a local function and then to twice apply it with the same set of arguments. On the whole, you make it all too complicated for no gain.

Why not write simply?:

projectileY g t v0 h0 = max 0.0 (-0.5 * g * (t ^ 2) + v0 * t + h0)

As another style note, in physical formulae, I find it much more readable to use conventional short identifiers for variables and constants, such as g, h and t.

Upvotes: 0

Sibi
Sibi

Reputation: 48746

If you change the type signature and do some cleaning up (proper indentation, not mixing tabs and spaces), it should work. This is an working code:

projectileY :: (Fractional a, Ord a) => a -> a -> a -> a -> a
projectileY gravity time vInit height = let equation gr ti veli hei = -0.5*g*(t^2)+vi*t+h
                                        in if (equation g t vi h < 0) 
                                           then 0 
                                           else (equation g t vi h)
                                               where
                                                 g = gravity
                                                 t = time
                                                 vi = vInit
                                                 h = height

main :: IO()
main = print (projectileY 9.8 0.1 2.0 100)

Am I allowed to use "in" and "where" like this?

Yes. But note that in should be used alongside with let. The let in syntax should form an expression.

Upvotes: 5

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