The nth term of the sequence of triangle numbers is given by, tn = ½n(n+1); so the first ten triangle numbers are:

1, 3, 6, 10, 15, 21, 28, 36, 45, 55, …

By converting each letter in a word to a number corresponding to its alphabetical position and adding these values we form a word value. For example, the word value for SKY is 19 + 11 + 25 = 55 = t10. If the word value is a triangle number then we shall call the word a triangle word.

Using words.txt (right click and ‘Save Link/Target As…’), a 16K text file containing nearly two-thousand common English words, how many are triangle words?

素直な方法、言われたことをそのまま実行。 

import Data.Char
triangle =[n*(n+1)`div`2|n<-[1..]]
isIn (x:xs) y | x == y = True
| x < y =isIn xs y
| x > y = False
value =sum.map (v.toUpper)
where v = flip (-) (ord 'A' - 1).ord
isTriangle = isIn triangle . value
main =getContents>>=print.length.filter isTriangle.(read::String->[String])