First Haskell program not launching

Question

I have written my first Haskell code in Visual Studio Code. I want to implement list of lists so I started with simple list.

module Main where
import System.Environment
import Data.List
import Data.Ord

main :: IO ()
main =  do
    let lostNumbers = [4,8,15,16,23,42]

When I try to launch my program I get:

How do I declare a list of lists in Haskell? Or at least just a simple list?

EDIT: I have found topic that discuss on where to look for Haskell tutorials: Getting started with Haskell

Hope it will help someone

You don't do anything in your `do`. The last statement of a `do` block can not be a `let`, since you simply define a local variable, and then throw it away, making the `do` block non-sensical. — Willem Van Onsem, Mar 28 '21 at 15:46
Thank you. Havent realised that since I am completely new :) — vytaute, Mar 28 '21 at 15:49

score 2 · Accepted Answer · answered Mar 28 '21 at 15:47

2

The error message tells you exactly what is wrong. The last line of your do block is that let expression and not a value of type IO ()

Update it to, for example:

lostNumbers = [4,8,15,16,23,42] 

main :: IO ()
main =  do
    print lostNumbers

If the let is important to you..

main :: IO ()
main =  do
    let lostNumbers = [4,8,15,16,23,42] in print lostNumbers

answered Mar 28 '21 at 15:47

Holp

Thank you, it helped. I am new. It is very hard to find any good tutorials about Haskell. Not even how to declare an array, I couldnt find it anywhere. Is it dead language, or I was looking in wrong places or what? :) – vytaute Mar 28 '21 at 15:51
2

There are excellent books for learning Haskell. For example https://haskellbook.com/ – Holp Mar 28 '21 at 15:53
3

[Learn You a Haskell](http://learnyouahaskell.com/) is by far the best for a Haskell beginner in my opinion. There's also the excellent [wikibook](https://en.wikibooks.org/wiki/Haskell) – Robin Zigmond Mar 28 '21 at 15:54
haskellbook.com is paid, but there are 95 pages for free so thanks :) – vytaute Mar 28 '21 at 15:55
Learn You a Haskell sure does cater more to beginner sensibilities. I'm not sure it's a better book for beginners than Haskell Book though. – Holp Mar 28 '21 at 15:56

score 1 · Answer 2 · answered Mar 28 '21 at 15:52

1

When I compile your code, I get this error:

main.hs:8:5: error:
    The last statement in a 'do' block must be an expression
      let lostNumbers = [4, ....]

So the last statement of the do block must be an expression such as

print lostNumbers

answered Mar 28 '21 at 15:52

Eloi Torrents

Thank you, maybe you also now how to declare array of arrays? I want to have array in which every element can be array of different sizes – vytaute Mar 28 '21 at 15:54
1

myListOfLists = [[1,2], [3,4]] – Holp Mar 28 '21 at 15:57

score 0 · Answer 3 · answered Apr 03 '21 at 18:40

your code have unused imports as a starting point to write haskell code, I modified your code to

lostNumbers :: [Integer]
lostNumbers = [4,8,15,16,23,42]

or, in case you would like to use module keyword, you have to be careful for spaces for example

module Main where
    lostNumbers :: [Integer]
    lostNumbers = [4,8,15,16,23,42]

3 Answers3