Haskell list within a list within a list

Question

I am learning Haskell and I have a problem where if given a list of a list of a list of Strings I have to find the total number of Strings in the entire initial list. My though process was to convert the entire initial list into a one dimensional list and get the length, however I can seem to figure it out.

What I have so far:

type Word = String
type Line = [Word]
type Page = [Line]
type Doc = [Page]
numWords :: Doc -> Line -> Word -> Int

I need to find the number of Words and I'm not sure if this is even the best way to go about this type of problem, but I am at a loss for ideas.

Any help would be much appreciated. Thank you.

AJF · Accepted Answer · 2015-04-11 18:19:38Z

1

This is quite a simple problem.

If we have a look at the types, it becomes easy:

type Word = String
type Line = [String]
type Page = [[String]]
type Doc  = [[[String]]]

It's not too difficult to get the length of a Line:

lenLine x = length x

The Page is just the sum of all the inner lengths:

lenPage xs = sum $ map lenLine xs

And we can use the same kind of pattern for Doc:

lenDoc xs = sum $ map lenPage xs

As for your final function , just use all the others:

numWords :: Doc -> Page -> Line -> Int
numWords dc pg ln = lenDoc dc + lenPage pg + lenLine ln

Remember that a type synonym doesn't mean that the type is actually different; you can use the same functions.

answered Apr 11, 2015 at 18:19

AJF

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4 Comments

Jon Over a year ago

Thank you so much! The data type stuff is very confusing for me as I'm coming from Java. It seems very different.

AJF Over a year ago

@Jon Don't worry about it - it grows on you :)

Nathan Over a year ago

Indeed it does grow on you. :) After using Haskell for a while, you'll find Java more and more unwieldy.

Nathan Over a year ago

@Jon keep in mind that having numWords take a document and a page and a line might be triple-counting some lines depending on what you pass it. The simplest way is to just use the lenDoc function AJ gave, unless you have some specific need to do something else.

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