Question

[(a,b) | a <- [1..5], b <- [1..5], a+b <- [1..10] ]

Trying to define define a list that follows these rules. I know it doesn’t allow the way I am adding a and b but I don’t understand why.

Edited forgot the “<-“

Answer 1

Maybe you want this?

[(a,b) | a <- [1..5], b <- [1..5], a + b >= 1 && a + b <= 10]

Or this?

[(a,b) | a <- [1..5], b <- [1..5], a + b `elem` [1..10]]

Haskell doesn’t solve equations for you, it performs calculations.

This declares the values which a variable takes:

a <- [1..5]

This is a request for Haskell to solve an equation, which it doesn’t do… the left side has to be a valid pattern.

a + b <- [1..10] # Not valid Haskell

Of course, patterns can be more sophisticated,

> [a | Just a <- [Just 10, Nothing, Just 20]]
[10, 20]