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| diff --git a/content/learn/06.pointers/default.md b/content/learn/06.pointers/default.md new file mode 100644 index 0000000..a4108bb --- /dev/null +++ b/content/learn/06.pointers/default.md @@ -0,0 +1,171 @@ +--- +menuTitle: Pointers +title: "Addressing Pointers" +weight: 6 +--- +In [the previous step](/learn/files), we split the story into multiple files +to make it more readable. Now, we'll see pointers, because they were needed for +two features that were glossed over in what was presented before: +* Using sequences as imperative procedures. +* Prompting the user for content. + +A pointer is a value that indicates a location in memory where some +data is stored. This can be used as a value that will tell some instruction +where to put data. + +Pointers have types. For example, `(ptr int)` is the type corresponding to +pointers to `int` data. + +To compute a pointer to a variable `v`, simply write `(ptr v)`. + +Given a pointer `p`, the variable being pointed to can be referred to using +`(at p)`. + +**data.fate:** + +         (fate_version 1) + +         (global int hero_money) +         (global string hero_name) + +         (set hero_money 42) + +**get_a_refill.fate:** + +         (fate_version 1) + +         (require data.fate) +         (require actions.fate) + +         (define_sequence lower_price_of_booze +            ( +               ((ptr int) price_pointer) +               (int decrease) +            ) +            Great! The price of booze just lowered from (at price_pointer) +            (set (at price_pointer) +               (- +                  (at price_pointer) +                  (var decrease) +               ) +            ) +            to (at price_pointer)! +         ) + +         (define_sequence get_a_refill () +            (local int price_of_booze) + +            (set price_of_booze 12) + +            Staring straight at the barman, you raise your glass and proclaim: +            (newline) +            "This soon-to-be world savior needs more booze!" +            (newline) +            The barman's lack of reaction is disappointing, but seeing the beer +            being poured does help improve the mood. +            (newline) +            Satisfied, you hand the barman (var price_of_booze) copper coins. +            (visit pay (var price_of_booze)) +            (newline) +            The barman sighs, then asks: +            (prompt_string (ptr hero_name) 2 64 What is your name, then, hero?) +            (var hero_name)? +            (newline) +            The barman looks surprised. +            (newline) +            (visit lower_price_of_booze (ptr price_of_booze) 4) +            (newline) +            "I have heard of you, (var hero_name)," the barman exclaims, "I have +            a quest for you!" +            (newline) +            It's your turn to sigh. +            (newline) +            The barman hands you a bag, and says: +            (newline) +            "Take this pre-payment and head to the smithy." +            (newline) +         ) + +* `(prompt_string (ptr hero_name) 2 64 What is your name, then, hero?)` prompts +  the player with `What is your name, then, hero?` and expects a string input +  with a size between `2` and `64` characters. This input is stored at `(ptr +  hero_name)`, which means that `hero_name` takes that value. +* The `lower_price_of_booze` sequence shows how pointers can be used to modify +  variables outside of a sequence's range. +* `(var price_of_booze)` is equivalent to `(at (ptr price_of_booze))`. + +Our hero, who'll obviously end up being the lost heir of some royal family, +should already have good equipment. It would be useful to have a character +sheet, so [let's create one](/learn/structures). + +---- + +## Unchanged Files +**actions.fate:** + +         (fate_version 1) + +         (require data.fate) + +         (define_sequence pay ( (int cost) ) +            (set hero_money +               (- (var hero_money) (var cost)) +            ) +         ) + +**falling_asleep.fate:** + +         (fate_version 1) + +         (require data.fate) + +         (define_sequence fall_asleep () +            Deciding to break away from the expected storyline, you promptly +            fall asleep. +            (newline) +            ... +            (newline) +            Upon waking up, your hard-trained reflexes inform you that someone +            stole all your money. +            (set hero_money 0) +            (newline) +            This set-back was more than you could take. You give up on this +            barely coherent story. +            (end) +         ) + +**main.fate:** + +         (fate_version 1) + +         Once upon a time, starting a story with these words wasn't considered +         a cliche. Starting in a tavern might also not be seen as very +         original.  Having the main character be an street orphan, raised by +         some mysterious sage all to end up as a mercenary with an uncommonly +         strong sense of honor probably isn't going to lead to any praises for +         novelty either. Maybe you should drink to that. +         (newline) +         Or maybe you shouldn't. This isn't your first mug. Not your second +         either.  Drinking to forget that you are a stereotypical hero isn't +         going to solve anything. Worse, the alcoholic trait is part of the +         image. +         (newline) +         As you contemplate your own pointless description, your gaze leaves +         what turns out to be an already empty glass in your hand and finds the +         barman. + +         (player_choice +            ( +               ( Ask the barman for a refill ) +               (visit get_a_refill) +            ) +            ( +               ( Fall asleep ) +               (jump_to fall_asleep) +            ) +         ) + +         (require get_a_refill.fate) +         (require falling_asleep.fate) + +         (end) | 


