How to use groupBy and zip in real practice?

import Data.List.Split
import Data.List(nub, groupBy)

z = splitOn "+" "x^2+2*x^3+x^2"

y = map (splitOn "*") z

x = map head y

toInt :: [String] -> [Int]
toInt = map read

u1 = filter ((< 2) . length) y
u2 = filter ((> 1) . length) y

v = map ("1" :) u1

q = u2 ++ v
q2 = zip toInt(map head q) (map last q)
q6 = groupBy nub(map tail q) q
q3 = map tail q
q5 = nub(q3)

q1 = map head q

1. For

zip toInt(map head q) (map last q)

I would like to add back the head to the tail after convert head int开发者_C百科o integer result should be [[1,"x^3"],[2,"x^2"],[1,"x^2"]]

I can do

*Main Data.List> zip [2,1,1] ["x^3","x^2","x^2"]
[(2,"x^3"),(1,"x^2"),(1,"x^2")]

but above can not, and there is a difference I noticed is, this is (), not []

2. How to write groupBy on a list, I have passed distinct elements for groupBy After grouping, it is for adding their head

groupBy (nub(map tail q)) q

:1:10: Couldn't match expected type a0 -> a0 -> Bool' with actual type[a1]' In the return type of a call of nub' In the first argument ofgroupBy', namely `(nub (map tail q))' In the expression: groupBy (nub (map tail q)) q

q is like a hash table, it seems that it can not group by second element

One issue is that zip toInt(map q) (map last q) isn't getting parsed the way you think it is.

Unlike languages with C-style syntax, haskell parses the above as

 zip toInt (map head q) (map last q)

(Note the space).

That is, it's not applying toInt to the result of map head q the way you want it to. Instead, it's attempting to do zip toInt (map head q), which will give you a type error, since you're zipping a function and a list.

What you want instead is

 zip (toInt (map head q)) (map last q)

Or slightly more succinctly

 zip (toInt $ map head q) (map last q)

As for your second issue, you're having a similar issue with syntax. Also, the first argument to groupBy needs to be a function that determines equality for the purposes of creating groups.