Nuthatch

Tree-Walking for Peeps

Example: Inserting Horizontal Space

  • Goal: insert/remove horizontal space according to rules

  • Basic idea: look at previous token and next token, decide whether to insert space in front of next token.

    • All existing spaces are removed, except those at the beginning of a line (after(START), after(NL)).

    • A list of rules deal with the particulars, for example:

      • We want space around binary operators, no space after a prefix operator and no space before a postfix operator.

      • No spaces around parentheses, braces and brackets.

      • No space before comma, always after.

      • etc.

    • By default, we always insert a space. The particular rules override this by specifying a ‘nop’ action instead.

    • Priority levels are used to make early rules take precedence over later rules. Also, rules for more specific token categories take precedence over generic categories.

Rule-Based Token Processor to Normalise Horizontal Spaces (SensibleSpacing.java) download
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package org.nuthatchery.pgf.examples;

import org.nuthatchery.pgf.rules.RuleProcessor;
import org.nuthatchery.pgf.rules.actions.Action;
import org.nuthatchery.pgf.tokens.Category;
import org.nuthatchery.pgf.tokens.CategoryStore;
import static org.nuthatchery.pgf.rules.PatternFactory.*;

public class SensibleSpacing extends RuleProcessor {

  @SuppressWarnings("unused")
  public SensibleSpacing(CategoryStore store) {
      super(store);
      boolean spaceBeforeCond = true;
      final Category TOKEN = store.category("TOKEN");
      final Category TXT = store.category("TXT");
      final Category SPC = store.category("SPC");
      final Category CTRL = store.category("CTRL");
      final Category START = store.category("START");
      final Category END = store.category("END");
      final Category WS = store.category("WS");
      final Category NL = store.category("NL");
      final Category COM = store.category("COM");
      final Category ID = store.category("ID");
      final Category LITERAL = store.category("LITERAL");
      final Category KEYWORD = store.category("KEYWORD");
      final Category OP = store.category("OP");
      final Category PREOP = store.category("PREOP");
      final Category BINOP = store.category("BINOP");
      final Category POSTOP = store.category("POSTOP");
      final Category GRP = store.category("GRP");
      final Category PAR = store.category("PAR");
      final Category BRC = store.category("BRC");
      final Category BRT = store.category("BRT");
      final Category LGRP = store.category("LGRP");
      final Category RGRP = store.category("RGRP");
      final Category LPAR = store.category("LPAR");
      final Category RPAR = store.category("RPAR");
      final Category LBRC = store.category("LBRC");
      final Category RBRC = store.category("RBRC");
      final Category LBRT = store.category("LBRT");
      final Category RBRT = store.category("RBRT");
      final Category PUNCT = store.category("PUNCT");
      final Category COMMA = store.category("COMMA");
      final Category SEMI = store.category("SEMI");
      final Category COLON = store.category("COLON");
      final Category DOT = store.category("DOT");
      Action space = insert(" ", WS);

      // spaces
      addRule(after(START).at(WS), nop);
      addRule(after(NL).at(WS), nop);
      addRule(at(WS), drop);

      addPriorityLevel();

      addRule(after(PREOP).at(TXT), nop);
      addRule(after(PREOP).at(OP), space);
      addRule(after(TXT).at(BINOP), space);
      addRule(after(BINOP).at(TXT), space);
      addRule(after(OP).at(POSTOP), space);
      addRule(after(TXT).at(POSTOP), nop);

      addPriorityLevel();

      addRule(after(COMMA).at(TXT), space);
      addRule(after(PUNCT), nop);
      addRule(at(PUNCT), nop);

      // no spaces around grouping tokens
      addRule(after(GRP), nop);
      addRule(at(GRP), nop);
      if(spaceBeforeCond) {
          addRule(after(KEYWORD).at(LGRP), space);
      }

      addRule(after(COLON).at(TXT), space);
      addRule(after(TXT).at(COLON), space);

      // Space around inline braces
      addRule(after(TXT).at(BRC), space);
      addRule(after(BRC).at(TXT), space);
      addRule(after(BRC).at(PUNCT), nop);
      addRule(after(ID).at(LBRC), nop);

      addRule(after(SEMI).at(TXT), space);

      addPriorityLevel();

      // general rule
      addRule(after(TXT).at(TXT), space);

  }
}