Inform 7
Adam Thornton Lambda Lounge 3 March 2011

Inform 7
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Domain-specific language for development of text adventures

Yawn.

Inform 7
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Declarative programming language With natural-language syntactic sugar Which is why it seemed that it might be interesting for Lambda Lounge Fun to try to wrap your head around; requires a different problem-solving approach than procedural, object-oriented, or functional languages

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Audience
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Most functional or dynamic languages we study are intended for experienced programmers
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Is Haskell anyone's first language? Fifth? In the proud tradition of BASIC, Applescript, and, yes, COBOL

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Inform 7 is designed for nonprogrammers
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Basic conceit: writing a text adventure (modern, pretentious term: “Interactive Fiction”; henceforth, “IF”) should feel like playing one

Audience
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So why are we looking at it?
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It's fun to look at a language designed for someone besides computer language geeks for a change Syntax is delightfully weird It's really fun and fast to write in, at least if you like Interactive Fiction
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If you don't, you probably want to go take a smoke break for the next half hour. Fair warning.

Hello World

World Model
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A text adventure is usually concerned with simulating a world, and then reasoning about objects and events within the simulated world. World state modeled with hierarchy of built-in and user-extended types. Changes in state modeled with actions invoked by rules applying to game state.

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Standard Rules
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Every game implicitly comes with “The Standard Rules”; this includes the object type hierarchy, 26 “activities” (e.g. “writing a paragraph about something”, “listing contents of something”), a bunch of standard textadventure actions (“taking”, “telling it about”, “waving hands”), and sequence-of-play rulebooks. All of these things can be overridden, replaced, or delisted as desired by the author.

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Built-in type hierarchy
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Object
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Direction Room Region Thing
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Supporter Backdrop Device Person
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Door Container
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Man Woman Animal

Vehicle Player's holdall

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That's all!

→ next column

Before we get rolling: heads-up
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Identifiers can have spaces in them.
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“Sasquatch's Den is a room.”

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What Inform calls a “kind”, most of us call a “class”:
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“A coin is a kind of thing.”

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We can define adjectives which are then usable in play and in world-construction:
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“Shininess is a kind of value. The shininesses are shiny and dull. A coin has a shininess. A coin is usually dull. The penny is a shiny coin in the Bank.”

Building the world model
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Done declaratively:
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Muddy Field is a room. Scotland is a region. Edinburgh, Glasgow and Aberdeen are rooms in Scotland. Aberdeen is northeast of Glasgow. Edinburgh is east of Glasgow. Glasgow is a room. “Gray and grim.” [ This sets the “initial appearance” property. ]

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Spatial relations (by default transitive)
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Populating the world
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Items are placed with declarations too:
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An ugly American is a man in Europe. The wooden table is a supporter in the kitchen. “A wobbly wooden table rests unsteadily on the floor.” The description is “The table looks unsteady.” Some butter is on the wooden table. The butter can be edible. It is edible. Understand “wobbly” and “unsteady” as the table.

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And properties:
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Synonyms:
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World model
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Certainly, it's a simulation
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But it's fundamentally a dramatic or narrative simulation, not a physical one The language pushes this view with its constructions: things not in any room are “offstage”, you “remove X from play” Default sense-modeling and object-player interaction scoping is primitive Other IF languages approach this differently: TADS 3 does very detailed sense-transmission modeling and more physics built into standard library

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Kinds
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Object, number, time, truth state, text, indexed text, snippet, Unicode character, stored action, scene, table name, equation name, use option, action name, figure name, sound name, external file “List of” for any kind K, and “List of lists of...” List of K, description of K, relation of K to L, K based rule producing L, K based rulebook producing L, activity on K, phrase K -> L, K valued property, K valued table column

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Adding Kinds
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Exactly what you'd expect. Either enumeration:
A sphere of dorkiness is a kind of value. The spheres of dorkiness are Trekkie, Star Wars Fanboy, Tentacle Hentai Enthusiast, and Joss Whedon Groupie.
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Or subclassing. And often both:
A nerd is a kind of person. A nerd has a sphere of dorkiness. A nerd is usually a Joss Whedon Groupie.

Adjectives
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Understood by parser and in world construction
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A coin is a kind of thing. A coin can be shiny or dull. A coin is usually dull. Understand the shiny property as referring to a coin.
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“referring to” makes it an adjective

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A metal is a kind of value. The metals are copper, silver, and gold. A coin has a metal. Understand the metal property as describing a coin.
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“describing” means you can use it as either a noun or an adjective

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The penny is a shiny copper coin in the bank.

Scoping
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Primitive. Everything is either a global, or a lexical local defined with “let”.
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It's easy to have confusing errors because you defined “the wooden table” but referred to it as “the table”, and the compiler thinks you're talking about “the table” you defined 20,000 words ago Still, works pretty well in practice, once you're done being surprised the first couple times. There's not much variable-passing, because typically things happen based on world-model state changes...so using globals actually kinda makes sense.

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Rules
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The world model type hierarchy lets us create the nouns and adjectives. Now we need to act on those things with verbs. Application of rules to world model. Rule conditions map onto sorta-kinda-like-aregex-on-the-whole-world-state Perhaps an example will make this more clear. From Ron Newcomb's Inform 7 for Programmers:

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Example rule
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Instead of a suspicious person (called the suspect) burning something which is evidence against the suspect when the number of people in the location is at least two, try the suspect going a random valid direction. Highlights: “Instead” is a rulebook name. “(called the suspect)” names a variable for reference later in the same rule. “Try” kicks off a new action and all its rulebooks. “Valid” is an adjective on class “Direction.”

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Rules
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Sequence of play rules
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“when play begins” “every turn” Before, instead, check, carry out, report, and after are the big ones

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Rules governing actions
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Preamble defines when rule is to be applied; code is what rule does. Preamble + code = rule

Example rules
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Check going up in Peak of Everest: instead say “There's no more 'up' from here.” Every turn when the nymph is in the location:
Let p be a random item enclosed by the player; Say “The nymph adroitly relieves you of [p]!”; Now p is held by the nymph.

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This is the stating the license rule:
say “This game is licensed under the Creative Commons 3.0 Attribution-NonCommercialShareAlike license.”

Adding new actions
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Define its arity, and come up with rules for its behavior. Really, the first part of this rule should be a “check” rule, not rolled into “report”

Rulebooks
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Every action gets a new Check, Report, and Carry out rulebook. Making a function call is a “to decide” rule (or “for deciding”. The thing that is decided on is the return value. “The fancy announce the score rule is listed instead of the announce the score rule in the carry out requesting the score rulebook.”

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Rulebooks

Relations
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Symmetric/asymmetric, one-to-one, one-tovarious, various-to-one, various-to-various. A role is a kind of thing. Characterization relates various roles to various people. The verb to be played by (he was played by) implies the characterization relation. Jennifer Aniston is a woman. Rachel is a role. Rachel was played by Jennifer Aniston.

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You can do most of this with tables too...which I find a lot easier.

Tables
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What they sound like. They have rows and columns; the things in a column are of the same type.
Choose a row in the Table of Real Cast Members with a Character entry of “Rachel”. If the met entry is true....

Syntax and Sugar
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Natural language; some people find it inherently offensive. I don't mind it. Pythonic or Algolic block constructs Looping: usually done with “repeat with x running through...” rather than an explicit loop variable. Lists: apply (==map), filter, reduce. But it's cheesy because there's no lazy evaluation and there's no such thing as an infinite list.

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More sugar
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Scenes – very useful for story purposes
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Slowly drowning begins when the location is the Lake. Slowly drowning ends happily when the lifeguard is in the location. Slowly drowning ends hellishly when the location is Hades. And then you can do things like Every turn during slowly drowning.... Can be nested, cannot overlap

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Regions: groups of locations
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WTF Language Features
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Dimensional Analysis:
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“A length is a kind of value. 10 m specifies a length. An area is a kind of value. 10 sq m specifies an area. A length times a length specifies an area. 10 cu m specifies a volume. A length times an area specifies a volume.”

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And even equation support Equation – Volume of a parallelepiped
V=lwh Where V is a volume, l is a length, w is a length, and h is a length

No, seriously, WTF ?
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Graham's a math professor; maybe there's some pedagogical aim here?

IDE and programmer support
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OS X, Windows, and Linux GNOME ports: nice IDE.
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Prettyprinting syntax and indentation Headings for code management: Volume, Book, Part, Chapter

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Comprehensive documentation with (in IDE) Javascript pasting from Recipe Book or doc examples into source text Skein for playthrough-path management

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IDE conveniences
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Automagic spatial index map (on Index tab) and as EPS Unit testing with the “test” command; also a way to fix-seed the PRNG for regression testing Comprehensive game index (Actions, Contents, Kinds, Phrasebook, Rules, Scenes, World) linked back into code Release along with....

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More IDE goodies
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Breakpoints and watchpoints, code stepper Transcript management, with “play to here” function, “bless” a transcript (great for regression testing), Skein integration ....or you can use the underlying ni (“natural Inform”, command-line tool) compiler directly and write your source text in the editor of your choice. Collaborative I7 at Guncho: www.guncho.com

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A snippet from a larger WIP
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About 160,000 words right now; release in Spring Thing 2011 (early April)

Under the Hood
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Inform 7 compiles to Inform 6 (a much more traditional OO language), which in turn compiles to z-code or glulx virtual machine. Inform 6:
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Sources: http://inform7.com/sources/i6n/ Manual: http://www.inform-fiction.org/manual/download_dm4.html

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Z-machine specs:
http://www.inform-fiction.org/zmachine/standards/index.html

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Glulx: http://eblong.com/zarf/glulx/

Inform 6

Inform 7 Source Code
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Implemented as a very large literate program
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In “Inweb”, a superset of a subset of CWEB, implemented in Perl. Actual code is ANSI C
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Requires GNU make, GCC 3+, Perl 5 Interfaces have more dependencies CLI requires Perl (or Python—XO port could be revived)

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Not yet publicly available as source; will be someday. In the meantime, the Standard Rules and Inweb are available as Literate Programs: http://inform7.com/sources/webs/

IF VMs
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Zcode: Infocom-format story file and virtual machine. 16-bit, 128K/256K/512K, depending on version. Glulx: 32-bit, a lot like zcode, but also separates presentation layer (into the glk API) Standalone zcode interpreters for damn near everything (including iPhone and Android) and glulx for most things modern (no mobile yet) Javascript terps; Android/iPhone doesn't quite work yet (displays, but no input). Fine in desktop browsers.

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zcode/glulx Interpreters
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Standalone:
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Gargoyle is my favorite:http://ccxvii.net/gargoyle/ Zoom's nice too: http://www.logicalshift.demon.co.uk/ Or just poke around on www.ifarchive.org Parchment (zcode): http://code.google.com/p/parchment/ Quixe (glulx): https://github.com/erkyrath/quixe Or just “release along with an interpreter”

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Javascript
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Inform 7 Ports
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Full GUI on Mac OS X, Windows, Linux i386/amd64 CLI only on Linux (i386,amd64, ppc, armv5tel, s390, s390x) and FreeBSD (i386, amd64) Used to be Solaris CLI ports too
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if you want it and can make hardware available for me I'm happy to revive it. I stopped when I changed jobs and lost access to the Solaris boxes. If you have Python but not Perl. Could revive.

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And XO
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Resources
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Inform and Extensions: http://www.inform7.com Ron Newcomb, Inform 7 for Programmers:
http://www.plover.net/~pscion/Inform%207%20for%20Programmers.pdf

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Aaron Reed,Creating Interactive Fiction with Inform 7 (ISBN 1435455061) Aaron Reed's Blue Lacuna source. 736 page PDF. Really.
http://www.lacunastory.com/BlueLacunaSourceBook.pdf.zip

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Resources
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rec.arts.int-fiction and rec.games.int-fiction IF Comp (www.ifcomp.org) , Spring Thing ( www.springthing.net) IF Archive: www.ifarchive.org IF Wiki: www.ifwiki.org Get Lamp: www.getlamp.com

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