PreParse

PreParse, as one might guess, is called by Parse and executed before the bulk of parsing is done.

In hugolib.h, it looks like this:

routine PreParse
{}

It is an empty canvas just waiting to be painted (replaced) by you! PreParse can be used for several techniques, some of which we will explore here.

Things to know about PreParse

When PreParse is called, object and xobject have not been set by the engine yet.
PreParse is only called if the player’s input line has been successfully matched to verb grammar. Otherwise, the engine calls ParseError directly.
When PreParse is called, the global verbroutine still equals the verb routine of the previous turn. It will be reset by the engine after Parse is finished.

Uses for PreParse

Spoon-feeding the Parser

Sometimes it’s hard to set up our verb grammar to properly understand every variation of a command. We can use PreParse to reorder commands into something our game will like better:

Kent Tessman’s Down

replace PreParse
{
    local i

    ! Since "get off wing" or "exit wing" will cause a parser complaint
    ! because the player isn't really "in" the wing, change either to
    ! simply "exit" (i.e., to direct the library to out_to).
    !
    if (word[1] = "get", "climb") and word[2] = "off"
    {
        word[1] = "exit"
        DeleteWord(2)
    }
    if word[1] = "exit" and ObjWord(word[2], wing)
        DeleteWord(2)

    ! Allow handing of, e.g., "ask girl about her mother", so that "her"
    ! doesn't get mapped incorrectly
    !
    if word[1] = "ask", "tell"
    {
        for (i=2; i<=words and word[i]~=""; i++)
        {
            if word[i] = "his", "her", "your"
            {
                DeleteWord(i)
                break
            }
        }
    }
}

In another example, the game understands the problem command just fine. We just don’t like the result it gives.

John Menichelli’s HugoZork

! This routine replaces "~all" with "grue" - if "~all" is passed through
! while in a dark location or while the player is a ghost, multiple
! "It's too dark to see!" or "Your hand passes through the object." messages
! will be printed; one for each object in the location. I replaced it with
! "grue" because the grue is a scenery item that is always in the player's
! location and thus can always be referred to.

replace PreParse
{
        if (not FindLight(location) or
           player is ghost) and
           word[1] ~= "drop"
        {
                if word[2] = "~all"
                {
                        word[2] = "grue"
                }
                elseif word[3] = "~all"
                {
                        word[3] = "grue"
                }
        }
        return false
}

Other Methods of Using Pre-Parse

Without even touching PreParse itself, you can also use PreParse to execute code each turn by adding code to the player_character object.

Kent Tessman’s Future Boy!

        before
        {
        actor PreParse
        {
            ! Reset things that need resetting each turn

            ! SOUND_STOP here so that, for instance, dialogue doesn't
            ! continue for several turns if the player keeps typing
            !if not just_played_sound
            !   PlaySound(SOUND_STOP)
            just_played_sound = false

            if display.needs_repaint
            {
                CheckIndentSize

                if DisplayHasGraphics
                {
                    ClearFullScreen
                    PrintStatusline
                    if DisplayHasGraphics
                        ForceAllPanelRepaint
                }

                display.needs_repaint = false
            }
            return false
        }

Notice that it returns false so any PreParse routine code is also run.

Important Note

If you add or subtract words from the word array, be sure to update the words global or else Parse may not function correctly.

So yeah, PreParse is useful indeed. Remember our slogan- “Got a problem the parser can’t handle? That’s ‘PreParse’-terous!”