Re: Functional design question

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    <p>Perhaps</p>
    <pre>DirectoryConfig&lt;C&gt; {
  Pair&lt;Path,C&gt; path
  Pair&lt;String,C&gt; file
}

Context {
  String filename
  int lineNumber, columnNumber
}</pre>
    <p><font face=3D"monospace">Pair</font> is a 2-tuple type.</p>
    <p><font face=3D"monospace">Unit</font> is an informationless type.</=
p>
    <p>Then you can have <font face=3D"monospace">DirectoryConfig&lt;Cont=
ext&gt;</font>
      and <font face=3D"monospace">DirectoryConfig&lt;Unit&gt;</font>.</p=
>
    <div class=3D"moz-cite-prefix">On 3/22/26 19:10, Joachim Durchholz
      wrote:<br>
    </div>
    <blockquote type=3D"cite"
      cite=3D"mid:[email protected]">Hi
      all,
      <br>
      <br>
      I have some design questions - not for Haskell but for the
      Java+Vavr combo, but I am about to dip my toes into actual work in
      a functional manner, as far as that is possible, and I thought I
      might as well go for the community that's best subscribed to a
      purely functional style.
      <br>
      <br>
      Bird's eye view:
      <br>
      <br>
      Java everybody knows (I don't like it either, no worries), Vavr is
      a - to my eyes - pretty nice functional library,
      <a class=3D"moz-txt-link-freetext" href=3D"https://docs.vavr.io/">h=
ttps://docs.vavr.io/</a> for details if you're actually interested.
      <br>
      I want to leverage Vavr to get a Java program to be as Haskell-ish
      as is reasonable; that's not going to be much by a Haskeller's
      standards, but... baby steps.
      <br>
      <br>
      Application is a simple command-line thing: Read configuration
      from command line and configuration file, emit diagnostics about
      any errors in the config, then process.
      <br>
      I want the configuration processing to be as functional as
      reasonable, given the constraints. However, I'm undecides about
      many things, no doubt because I simply don't know the best design
      patterns, and it's frustrating to see multiple options and not
      knowing which ones will paint me into a corner and which ones will
      not.
      <br>
      <br>
      Things I'm undecided about:
      <br>
      <br>
      a) Data type variations
      <br>
      During the configuration phase, I need to carry information about
      where some configuration item came from (its ("context", usually
      file, line number, column number).
      <br>
      In the processing phase, configuration is considered final and
      error-free, so context is not needed (that's a done design
      decision).
      <br>
      I could carry context information into the processing phase, but
      it's going to be awkward: Say, we have the following types
      (forgive the most un-Haskellish pseudo syntax but I don't dare to
      use Haskell style because I'd almost certainly get that wrong and
      provoke misunderstandings)
      <br>
      =C2=A0 DirectoryConfig {
      <br>
      =C2=A0=C2=A0=C2=A0 ConfigData&lt;Path&gt; path
      <br>
      =C2=A0=C2=A0=C2=A0 ConfigData&lt;String&gt; title
      <br>
      =C2=A0=C2=A0=C2=A0 ...
      <br>
      =C2=A0 }
      <br>
      =C2=A0 data ConfigData&lt;a&gt; {
      <br>
      =C2=A0=C2=A0=C2=A0 String fileName
      <br>
      =C2=A0=C2=A0=C2=A0 int lineNumber
      <br>
      =C2=A0=C2=A0=C2=A0 int columnNumber
      <br>
      =C2=A0=C2=A0=C2=A0 a value
      <br>
      =C2=A0 }
      <br>
      but in the processing phase I don't want my config objects
      polluted with context, so I want
      <br>
      =C2=A0 DirectoryConfig {
      <br>
      =C2=A0=C2=A0=C2=A0 Path path
      <br>
      =C2=A0=C2=A0=C2=A0 String title
      <br>
      =C2=A0=C2=A0=C2=A0 ...
      <br>
      =C2=A0 }
      <br>
      No idea how to deal with that. I'd use code generation in Java I
      guess, but that's horribly inelegant and complicated to set up
      (no, I don't particularly like Java, it's just what I'm currently
      using).
      <br>
      So... how would one do such a thing in a functional language?
      <br>
      Not necessarily Haskell, I guess some language extensions exist
      for that kind of stuff, but I'm more-or-less tied to Java +
      functional libraries, so I'll have to stick with the more basic
      approaches most likely. Besides, even if I did Haskell, I'd want
      to avoid the advanced stuff until I get confident in the basics.
      <br>
      <br>
      TL;DR: I have a deeply nested configuration data structure where
      each field has a "context", i.e. the place it came from; how to I
      make it so that the context is available during configuration
      evaluation but is unavailable in the later processing phase?
      <br>
      <br>
      I hope this is understandable; it's really hard to do that when
      you don't even know enough to ask the questions precisely enough.
      <br>
      <br>
      Regards,
      <br>
      Jo
      <br>
      _______________________________________________
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    </blockquote>
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