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Erro a utilizar divisão


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eu tenho o seguinte codigo:

--turma: [(1,"Passos Coelho",4.0,3.0),(2,"Miguel Caraças",10.0,6.0),(3,"Fernando Pessoa",8.0,3.0),(4,"Zé do Pipo",4.0,2.0)]
type Aluno = (Numero,Nome,ParteI,ParteII)
type Nome = String
type Numero = Int
type ParteI = Float
type ParteII = Float
type Turma = [Aluno]
--b) testar alunos que passaram
alunosPositiva:: Turma -> Turma
alunosPositiva (x:xs) = if alunoPassou x then x:alunosPositiva xs
 else alunosPositiva xs
alunosPositiva [] = []
alunoPassou:: Aluno -> Bool
alunoPassou (_,_,a,b) = (a>=8) && (a+b >= 9.5)
type NotaFinal = (Numero,String,Float)
notafinal:: Turma -> [NotaFinal]
notafinal t = notafinalAux $ alunosPositiva t
notafinalAux:: Turma -> [NotaFinal]
notafinalAux ((c,n,a,b):xs) = (c,n,a+b):notafinalAux xs
notafinalAux [] = []

o problema está aqui:

--d) media dos alunos que passaram
mediaFinal:: Turma -> Float
mediaFinal t = (sum (map getnota (notafinal t))) / (length (notafinal t))

getnota:: NotaFinal -> Float
getnota (_,_,n) = n

quando fasso o load para o ghci dá-me

Couldn't match expected type `Float' with actual type `Int'
In the return type of a call of `length'
In the second argument of `(/)', namely `(length (notafinal t))'
In the expression:
 (sum (map getnota (notafinal t))) / (length (notafinal t))

Se fizer separadamente o "length (notafinal t)" da-me 2 tudo ok - não percebo porque não consegue dividir por 2 -.-

Cumprimentos 😉

Edited by Baderous
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