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6722 :: Preserve static value in recursive macro

• aachh (orginal) [2020-08-24T17:20:10+02:00] view original

  Hey all! I'm trying to write a macro that given a seq[string], let's assume it's @["me", "location", "city"] for this example, would produce this output:

  globalObject["me"].value["location"].value["city"]
  Run

  I came up with this recursive macro:

  macro getFieldLocation(fields: static[seq[string]]): untyped =
    if fields.len() == 1:
      result = quote do:
        globalObject[`newLit(fields[0])`]
    else:
      let field = newLit(fields[^1])
      let leftSide = getFieldLocation(fields[0..^1])
      result = `leftSide`.value[`field`]
    
    echo result.repr
  Run

  The problem is when I try to recur, the compiler reports this error:

  
  Error: type mismatch: got <seq[string]>
  but expected one of:
  macro getFieldLocation(fields: static[seq[string]]): untyped
    first type mismatch at position: 1
    required type for fields: static[seq[string]]
    but expression 'fields[
    0 .. BackwardsIndex(1)]' is of type: seq[string]
  
  expression: getFieldLocation(fields[
    0 .. BackwardsIndex(1)])
  Run

  How can I preserve the "staticness" of the seq so I can recur with it safely? Is it possible at all?

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• lqdev (orginal) [2020-08-24T22:58:07+02:00] view original

  You probably want to split this macro into a separate proc that operates on a plain seq[string], and then make that recursive.

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• shirleyquirk (orginal) [2020-08-25T01:10:17+02:00] view original

  Something liiike

  import tables
  type
    Obj = ref object
      children: Table[string, Obj]
      value: string
  proc newObj():Obj=
    result = new Obj
    result.children=initTable[string, Obj]()
  proc newObj(key:string,kids:Obj=newObj()):Obj=
    result = new Obj
    result.children={key:kids}.toTable()
  proc `[]=`(o:Obj,k:string,val:string)=
    o.children[k].value=val
  proc `[]`(o:Obj,k:string):Obj=
    o.children[k]
  var rootobj = newObj("me",newObj("location",newObj("city")))
  rootobj["me"]["location"]["city"]="Toledo"
  
  
  proc getField(parent:Obj,f:seq[string]):string=
    if f.len==1:
      return parent[f[0]].value
    else:
      return getField(parent[f[0]],f[1..^1])
  
  echo getField(rootobj,@["me","location","city"])
  Run

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• aachh (orginal) [2020-08-25T12:45:40+02:00] view original

  Thank you all very much for replying! A slightly tweaked version of Shirley's second idea worked. I didn't figure I could use parseStmt() (or that it even existed :P). This is what I ended up with:

  macro getField(s: static seq[string]): untyped =
    var res = &"globalObject[\"{s[0]}\"]"
    
    for i in 1..<s.len():
      res &= ".value"
      res &= "[\"" & s[i] & "\"]"
    
    result = parseStmt(res)
  Run

  Again, thanks for helping!

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