Unexpected do block in function application involving ReadMode what does mean this message?

Question

This function reads a String and concatenates to a txt file

adicionaEmListaDeCategorias :: [Char] -> IO() 
adicionaEmListaDeCategorias categoria = do
    file <- openFile ("listagemCategorias.txt") ReadMode
    categorias <- hGetContents file
    let categoria1 = (read categorias :: [Char]) ++ categoria
    hClose file
    file2 <- openFile ("listagemCategorias.txt") WriteMode
    hPutStr file2 (categoria)
    hFlush file2
    hClose file2

but I'm receiving this error message:

main.hs:190:41: error:
    Unexpected do block in function application:
        do file <- openFile ("listagemCategorias.txt") ReadMode
    You could write it with parentheses
    Or perhaps you meant to enable BlockArguments?
    |
190 | adicionaEmListaDeCategorias categoria = do
    |                                         ^^...

Answer 1

as I said in the comment - the issue is that you had intended the first line with a tab

this one works:

adicionaEmListaDeCategorias :: [Char] -> IO()
adicionaEmListaDeCategorias categoria = do
    file <- openFile "listagemCategorias.txt" ReadMode
    categorias <- hGetContents file
    let categoria1 = (read categorias :: [Char]) ++ categoria
    hClose file
    file2 <- openFile "listagemCategorias.txt" WriteMode
    hPutStr file2 categoria
    hFlush file2
    hClose file2

but Haskell will give you an warning because you did not use categoria1 and I think you'll want this:

adicionaEmListaDeCategorias :: [Char] -> IO()
adicionaEmListaDeCategorias categoria = do
    file <- openFile "listagemCategorias.txt" ReadMode
    categorias <- hGetContents file
    let categoria1 = (read categorias :: [Char]) ++ categoria
    hClose file
    file2 <- openFile "listagemCategorias.txt" WriteMode
    hPutStr file2 categoria1
    hFlush file2
    hClose file2

(write the concatenated version)

---

also based on your problem description I think your version will not do what you expected (the read will fail and you don't need it):

adicionaEmListaDeCategorias :: [Char] -> IO()
adicionaEmListaDeCategorias categoria = do
    file <- openFile "listagemCategorias.txt" ReadMode
    fileContent <- hGetContents file
    hClose file
    let newFileContent = fileContent ++ categoria
    file2 <- openFile "listagemCategorias.txt" WriteMode
    hPutStr file2 newFileContent
    hFlush file2
    hClose file2