CODEWITHSUNDEEP
haskell
user4779
user4779

Reputation: 827

Simple sum function in Haskell from Graham Hutton's "Programming in Haskell" not working

On the first code example in "Programming in Haskell" the following is stated:

Recall the function sum used earlier in this chapter, which produces the sum of a list of numbers. In Haskell, sum can be defined using two equations:

sum [] = 0      
sum (n:ns) = n + sum ns

Immediately, this code fails both in the ghci interpreter and upon compilation with the error: "Non-exhaustive patterns in function sum"

After further research it seems this is because the case of a single number isn't covered. What gets me is the next few examples in the book also fail to work.

Am I missing something here? The book was released rather recently in 2016 and I can't find anyone else complaining aout this.

Upvotes: 3

Views: 675

Answers (1)

user156548
user156548

Reputation:

When you enter the first clause of the definition of sum and press enter GHCI assume that you've finished and it should evaluate the program. The program

sum [] = 0

Doesn't specify what to do if the argument is non-empty, so you get the "non-exhaustive patterns" error.

GHCI has :{ and :} commands to allow you to enter multiple clauses (or other code that spans multiple lines:

Prelude> :{
Prelude| sum [] = 0
Prelude| sum (n:ns) = n + sum ns
Prelude| :}
Prelude> sum []
0
Prelude>

In general I would recommend saving the definitions you are working with in a file and loading that file in GHCI. Then you can :reload when you make some changes and call your function/s with various arguments to see what happens.

Upvotes: 10

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