why can't I add Ints to the last Ints of a tuples?

Question

I have to a data Student and a type Class:

data Student = Student {nome :: String
                        , stdNumber :: Int
                        , approvedClass :: Int
                        , failedClass :: Int
                        }
type Class = [Student]

and I'm trying do add to the approvedClass number and failed of a Student but i'm don't know how to do it . I already have this:

addClasses :: Student-> Int -> Int -> Student
addClasses student aC fC = (student _ _ (approvedClass+aC) (faileClass+fC))

But it doesn't work and i can't understand why? or how to make it work?

score 7 · Accepted Answer · answered Nov 29 '20 at 16:10

_ is only valid in pattern matching, and does not automatically denote "keep this field the same". Instead, use record update syntax:

addClasses :: Student-> Int -> Int -> Student
addClasses student aC fC =
    student { approvedClass = approvedClass student + aC
            , failedClass = failedClass student + fC
            }

score 5 · Answer 2 · answered Nov 29 '20 at 16:15

5

As an alternative to record syntax, you would need to match each field explicitly to reuse in constructing a new Student value.

addClasses :: Student -> Int -> Int -> Student
addClasses (Student w x y z) aC fC = Student w x (y + aC) (z + fC)

answered Nov 29 '20 at 16:15

chepner

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why can't I add Ints to the last Ints of a tuples?

2 Answers2