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2142 :: Generating case statements with templates

• chrisheller (orginal) [2016-03-21T11:42:35+01:00] view original

  I have a scenario where I have an enum with a large number of values that is being generated by a macro from some external data. From the value of the enum, I have macro code that builds up the correct body of each ''of'' branch in the case statement.

  Imagine something like this being generated by a macro, except with a lot more values in the enum.

  type
    MyTest = mtSomething, mtOther, mtEtc
  
  proc doSomethingWithMyTest(mt: MyTest) =
    case mt:
      of mtSomething:
        discard "Some logic for mtSomething here"
      of mtOther:
        discard "Some other logic here"
      of mtEtc:
        discard "Etc, etc. etc"
  Run

  That all works perfectly, but I was wondering if I could simplify things a bit by having a template for defining the ''doSomethingWithMyTest'' proc.

  The main reason would just be for ease of reading since the real proc with the case statement in it has a bit more code to it. I generate the entire proc right now via the API, but I was wondering if there was any way to do something like this

  template makeMyProc() =
    proc doSomethingWithMyTest(mt: MyTest) =
      case mt:
        `something to insert the of branches here`
  Run

  Can I generate the ''of'' branches with the existing logic and then pass that into the template somehow?

  It's not a big deal since the macro API works well, but I was curious if this would be possible.

  Thanks

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• chrisheller (orginal) [2016-03-21T12:01:14+01:00] view original

  Oh yeah, I should probably mention one of the things that I tried when I was experimenting with this.

  template makeSkeleton() =
    ## Generates the skeleton AST for this proc.
    ## The caller is expected to fill in the case statement
    proc doSomethingWithMyTest(mt: MyTest) =
      case mt:
  
  macro makeTheWholeProc() : stmt =
    result = getAST(makeSkeleton())
    
    let caseStmt = result[^1][^1][0]
    
    # now use macro API to update the caseStmt NimNode with the ''of'' branches
  Run

  Grabbing the AST from a template with just the stub of the case statement does work, but this seems a bit fragile.

  Maybe this approach would be OK if I created some helper procs for walking the AST to find the case statement in more robust way than relying on indexes like this example is doing.

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• flyx (orginal) [2016-03-21T13:32:21+01:00] view original

  Apparently, inputting standalone of branches into a template does not work. You could do something like this, although I do not see much of an advantage:

  type MyTest = enum
    mtSomething, mtOther, mtEtc
  
  template makeMyProc(actionSomething, actionOther, actionEtc: stmt): stmt =
    proc doSomethingWithMyTest(mt: MyTest) =
      case mt
      of mtSomething: actionSomething
      of mtOther: actionOther
      of mtEtc: actionEtc
  
  makeMyProc() do:
    discard # mtSomething
  do:
    discard # mtOther
  do:
    discard # mtEtc
  Run

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• chrisheller (orginal) [2016-03-21T14:42:43+01:00] view original

  Thanks for the input flyx. I hadn't realized that you could pass more than one block to a template, so that's useful to know. I can't use it in this scenario since the values for the enum are generated from external data, but I can think of some other places where it might be handy.

  By the way, in case anyone else is interested in this sort of thing, here is a helper proc that I came up with for inserting the of branches into the case statement via the API.

  proc insertOfBranch(caseStmt, ofBranch: NimNode) {.compileTime.} =
    ## Adds ''ofBranch'' to the ''caseStmt''.  The ''ofBranch'' is inserted
    ## at the very end of the list, prior to an ''else'' branch, if there is one
    caseStmt.expectKind(nnkCaseStmt)
    ofBranch.expectKind(nnkOfBranch)
    if caseStmt.len <= 1:
      caseStmt.add(ofBranch)
    else:
      var insertPos = caseStmt.len
      if caseStmt[^1].kind == nnkElse:
        insertPos = caseStmt.len - 1
      caseStmt.insert(insertPos, ofBranch)
  Run

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