[Haskell-beginners] Design help to add imports to a (Java) template

[Hilco Wijbenga]

Hi all,

I'm working on a Haskell (Eta) tool to generate Java ADTs.

I have the following definitions:

data Line = Line Int Text [JavaImport] -- The Int represents the
indentation level of this line.
data Template
    = None
    | One Line
    | Many [Line]
    | Template (Reader Configuration Template)

I then run it like this (incomplete, I hope you get the idea):

main :: IO ()
main = putStr $ replace (importsGoHereKey `append` "\n") importsAsText
templateAsText
  where
    config = Configuration (Copyright 2018 "Me") USE_GETTERS
    point' = javaClassTemplate point
    importsAsText = runReader (mapReader javaImportsToText
(javaImports point')) config
    templateAsText = runReader (toText tab point') config

Essentially, I use the template twice: once to extract the Java
imports and once to generate the result Text. Then I replace a magic
string in the result Text with the import code. It works but makes me
sad. ;-)

I will probably pull out the JavaImport list from Line and use
something like State to keep track of the necessary imports. (Now that
I understand Reader, maybe State is no longer a bridge too far.)

My main issue is how to avoid the magic string. I thought about adding
an extra entry to Template [something like Imports ((Set JavaImport)
-> Text or maybe Template)] but then I don't know how to implement
"instance Monoid Template". Similarly if I use something Reader like.

Any design advice or ideas would be appreciated. If it's of any help,
the full code is at [1] (it's tiny). Any and all comments are welcome,
by the way: I'm still a Haskell beginner.

Cheers,
Hilco

[1] https://github.com/hilcode/java-algebraic-data-types