## Working design

The CTPL engine is split, as most parsers, in two distinct parts: the lexer and the parser.

### The lexer

The lexer is the part that reads the actual input data, and tries to create a token tree (internal representation of the input) that the parser will use.

### The parser

The parser reads a token tree and an environment, and outputs the computed template.

By exposing this separation of the tasks to the user, it is possible to parse a single template many times with a different environment without needing to re-lex it, which will save computation time and resources.

Example 1. Using the library to lex and parse a template

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    #include <ctpl/ctpl.h>

    /**
     * procede_template:
     * @input:  The input stream
     * @env:    The environment against which parse the input.
     * @output: The output stream
     * @error:  Return location for an error, or %NULL to ignore them.
     *
     * Parses and lexes a template.
     *
     * Returns: %TRUE on success, %FALSE on error.
     */
    gboolean
    procede_template (CtplInputStream  *input,
                      CtplEnviron      *env,
                      CtplOutputStream *output,
                      GError          **error)
    {
      gboolean    success = FALSE;
      CtplToken  *tree;

      /* first, create a token tree from the input template */
      tree = ctpl_lexer_lex (input, error);
      if (tree != NULL) {
        /* then, parse this tree against the environment */
        success = ctpl_parser_parse (tree, env, output, error);
        /* and finally, free the built tree */
        ctpl_lexer_free_tree (tree);
      }

      return success;
    }