Adam Thornton

Created: 2024-04-27 Sat 09:14

A brief introduction to the Inform 7 text adventure programming language

Adam Thornton, April 2020

https://github.com/athornton/i7-talk-2020

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CC BY-NC 4.0

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Inform 7

by Graham Nelson, after Don Knuth

…and Crowther and Woods, and Anderson, Blank, Lebling, and Daniels

…and Roberts and Tessman and Plotkin

…and a host of others.

Parser-based Text Adventures

For a brief shining moment, the most popular form of computer entertainment.

starcross.jpg

Get offa my lawn!

…and a very, very long post-commercial life (1990-present)…

"Interactive fiction."

…frequently used to storyboard games that will have expensive assets.

Good way to prototype puzzle design, for instance.

How were these written, historically?

Adventure: FORTRAN/PDP-10/TOPS-10

KI-10.jpg

http://www.literateprogramming.com/adventure.pdf may be the best paper ever written about a computer program.

(KI-10 Picture from Wikipedia user Gah4, CC BY-SA 4.0)

Zork: MDL/PDP-10/ITS

https://github.com/historicalsource/zork

Ported to FORTRAN by Bob Supnik. That's the one you've played.

Infocom: ZIL/PDP-10/TOPS-20

https://github.com/historicalsource/

The Z-Machine was the magic that enabled easy porting and very big (for the time) games.

Post-Infocom: AGT, TADS

  • Even later: Hugo, ChoiceScript, Twine…

Inform: Graham Nelson, 1993 -> Inform 6, 1996

Inform 7: Graham Nelson, 2006

Compilation Target

Z-machine: 16-bit virtual machine

  • Most Infocom games version 3 (<= 128K)
  • Late Infocom games version 5 (<= 256K)
  • Little-used graphical variant version 6 (<= 256K)
  • Post-Infocom version 8 (<= 512K)
  • Finally, Glulx (32-bit Z-Machine-inspired VM, Plotkin 1999)
  • I7 produces Z8 or Glulx

What's it like?

The experience of writing an adventure game should be much like the experience of playing one.

Designed for nonprogrammers:

  • In the tradition of BASIC, Hypercard, Scratch…
  • …and COBOL.

Graham on Inform 7 design:

http://inform7.com/talks/2018/06/09/london.html

Literate Programming

  • I need to talk to Graham about Jupyter as a resurgence of LP.

Declarative

Hello, World

"Hello World" by Adam Thornton.
Hello World is a room.

Note that identifiers can have spaces (and other odd characters) in them.

IDE is an integral part of the intended experience

I7UI.png

But not, strictly speaking, a necessary one.

I maintain the Linux CLI port.

However, the IDE is a joy to use.

  • Integrated documentation, both reference and recipe book
  • Testing panel lets you do regression testing and diverging-output-at-nodes
  • Excellent indexing facility with automapping
  • Good source-level debugger

How suitable is it for writing text adventures?

I have written a 160,000 word game in it.

  • https://www.stiffymakane.com/MMA
    • WARNING: NOT SAFE FOR WORK. NOT KIDDING.
    • 160,000 word pornographic text adventure, 175 rooms, 735 things…
    • …set in the waning days of the Roman Republic.
    • What?

Blue Lacuna is about twice that size(!)

Far, far more output text than any commercial text adventure ever.

Find Inform 7 at http://inform7.com

Not Open Source yet (although it was announced for last fall)….

Inform 7 itself is a very large literate program, written in Inweb (a superset of a subset of CWEB)

https://github.com/ganelson (someday)

What does it simulate?

A physical world

Rooms are topologically connected, there are objects, some of which are mobile…

But more like a stage-set than a physical simulation

The language encourages this: objects are "off-stage", "remove X from play," and sense-modelling and object-player interaction are primitive. Cf. TADS 3.

Language features

Locations defined declaratively, implicitly transitive.

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. ]

Populating the world is declarative:

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."
Understand “wobbly” and “unsteady” as the table. [ Synonyms ]
Some butter is on the wooden table.  The butter can be edible. It is edible. [ Properties ]

Adjectives used in play and in world-construction.

[ Define a new kind, and then use it as an adjective. ]
Shininess is a kind of value.  The shininesses are shiny and dull.

A coin has a shininess.  A coin is usually dull.

The Bank is a room.  The penny is a shiny coin in the Bank.

Defining new actions

Understand the command "feed" as something new.  Understand "feed [something preferably held] to [something]" as feeding it to.  Understand "feed [something] [something preferably held]" as feeding it to (with nouns reversed).
Feeding it to is an action applying to two things.
Carry out feeding it to:
        if the second noun is not a person, instead try inserting the noun into the second noun;
        if the second noun is the player, instead try eating the noun;
        instead try giving the noun to the second noun.

Rule-based

The most important ones are "before", "instead", "after", and "check <action>", "carry out <action>", "report <action>".

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.
[ "Instead" is the rulebook name; "(called the suspect)" creates a scoped variable for reference within the same rule.  "Try" kicks off a new action and all its rulebooks.  "Valid" is an adjective applying to the kind "Direction". ]

Implicit loop variables:

For printing a locale paragraph about a thing (called the item) (this is the forcibly set personal pronoun from items on supporters rule):
        if the item is a supporter and the item does not enclose the player
        begin;
                repeat with the possibility running through things on the item
                begin;
                        if the possibility is a woman, forcibly set the female pronoun from the possibility;
                        if the possibility is a man, forcibly set the male pronoun from the possibility;
                        if the possibility is a neuter animal, forcibly set the neuter pronoun from the possibility;
                end repeat;
        end if;
        continue the activity.

You can also, if you prefer, use Python semantic indentation rather than "begin/end".

Tables take the role of structs.

table.png

  • Rows and columns
  • Things in a column are of the same type.

Lists support apply, filter, and reduce…but not lazy evaluation.

Dimensional analysis (what?)

"Equation Playground" by Adam Thornton

Part Zero - Definitions

Include Metric Units by Graham Nelson.

Part e - Equations

Equation - Volume of a square parallelepiped
        V=hl^2
Where V is a volume, h is a length, and l is a length.

Equation - Area of a square
        A=l^2
Where A is an area and l is a length.

Part pi - Objects

Classroom is a room

The infernal prism is a thing in Classroom.  It is fixed in place.

Carry out examining the infernal prism:
        Let V be a random volume between 10 cu m and 1000 cu m;
        Let A be a random area between 10 sq m and 100 sq m;
        let l be given by the area of a square;
        let h be given by the volume of a square parallelepiped;
        say "The infernal prism shifts again.  Now its height is [h].  Somehow you know its volume is [V] , so the side of its base must be [l]  and the area of its base [A].";
        stop the action.

A more traditional programming approach to I7:

Ron Newcomb, http://www.plover.net/~pscion/Inform%207%20for%20Programmers.pdf

May help impedance-match if you're more used to coding than writing.

Changes coming in the open-source version, whenever that may be

Two of Graham's talks cover a lot of this

http://inform7.com/talks/2018/06/09/london.html http://inform7.com/talks/2019/06/14/narrascope.html

LLVM-inspired intermediate representation ("inter")

  • Compile to Inform 6 (status quo), or C, or Javascript, or Unity (!!)

Give it a try!

It's fun to try just plain strange languages sometimes.

FRACTRAN

  • A starting integer n, and an ordered list of fractions.
  • For the first fraction f for which nf is an integer, replace n by nf and repeat.
  • When no nf is an integer, halt.