Applying a function with multiple inputs using Map? (Haskell)

Question

G'day guys,

Trying currently to finish up a bit of homework I'm working on, and having an issue where I'm trying to apply map across a function that accepts multiple inputs.

so in the case I'm using processList f (x:xs) = map accelerateList f xs x xs

processList is given a floating value (f) and a List that it sorts into another List

Accelerate List takes a floating value (f) a List and a List Object through which it returns another List Object

I know my Accelerate List code is correct, but I cannot for the life of me get the syntax for this code working:

processList :: Float -> [Object] -> [Object]
accelerate f [] = []
accelerate f [x] = [(accelerateForce f x x)]
accelerate f (x:xs) = map accelerateList f xs x xs

Any ideas? I've been scratching my head for about 3 hours now. I know it's something really simple.

Jakob · Accepted Answer · 2010-05-16 09:29:37Z

5

First of you will probably want to use some parenthesis here:

map accelerateList f xs x xs

The function map takes exactly two arguments (not five), so you should do something like this for example:

map (accelerateList f xs x) xs

But on the other hand, that doesn't fit with your function signatures. The problem is probably that you haven't structured your solution in a good enough way. Might be a separate question, but explaining what you're trying to accomplish with the accelerate-function (or which ever one is the "top" one) would certainly help.

answered May 16, 2010 at 9:29

Jakob

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2 Comments

Schroedinger Over a year ago

I absolutely thought it was a structure error, thank you. I'm attempting to map a function that compares two elements in a list, across an entire list of objects. So for each object I have to compare it to each other object in the list to calculate a motion for it.

Schroedinger Over a year ago

With a bit of buggerising around I actually got map working properly, and the function that I've tested it on works appropriately as well: accelerate f (x:xs) = map (accelerateList f xs) xs Works wonderfully. Thanks for the help guys!

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