Inform Primitives

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Inform Primitives

The standard set of primitive invocations used within Inform.

§1. Status. The Inter specification allows for any number of primitive invocations to be declared and used; none are built-in or required.

The Inform compiler, however, has a set of around 90 different primitives. The back end of the compiler can compile those into valid Inform 6 code; the front end of the compiler is guaranteed to declare and use only (a subset of) those 90. That gives the following set of primitives a kind of halfway status: though they are not part of the inter specification, for the time being they're the only game in town.

§2. Arithmetic. The following are standard integer arithmetic operations:

(a) primitive !plus val val -> val. 16 or 32-bit integer addition.
(b) primitive !minus val val -> val. 16 or 32-bit integer subtraction.
(c) primitive !unaryminus val -> val. Equivalent to performing 0 - x.
(d) primitive !times val val -> val. 16 or 32-bit integer multiplication.
(e) primitive !divide val val -> val. 16 or 32-bit integer division.
(f) primitive !modulo val val -> val. Remainder after such a division.

§3. Logical operators. In general, the value 0 is false, and all other values are true.

(a) primitive !not val -> val. True if the value is false, and vice versa.
(b) primitive !and val val -> val. True if both are true: doesn't evaluate the second if the first is false.
(c) primitive !or val val -> val. True if either is true: doesn't evaluate the second if the first is true.

§4. Bitwise operators. These differ in that they do not "short circuit", and do not squash values down to just 0 or 1.

(a) primitive !bitwiseand val val -> val.
(a) primitive !bitwiseor val val -> val.
(a) primitive !bitwisenot val -> val.

§5. Numerical comparison. These are comparisons of signed integers. (If Inform needs to compare unsigned integers, it calls a routine in the I6 template.)

(a) primitive !eq val val -> val.
(b) primitive !ne val val -> val.
(c) primitive !gt val val -> val.
(d) primitive !ge val val -> val.
(e) primitive !lt val val -> val.
(f) primitive !le val val -> val.

§6. Sequential evaluation. The reason for the existence of !ternarysequential is that it's a convenient shorthand, and also that it helps the code generator with I6 generation, because I6 has problems with the syntax of complicated sequential evals.

(a) primitive !sequential val val -> val. Evaluates the first, then the second value, producing that second value.
(a) primitive !ternarysequential val val val -> val. Evaluates the first, then the second, then the third value, producing that third value.

§7. Random. This is essentially the built-in random function of Inform 6, given an Inter disguise. See the Inform 6 Designer's Manual for a specification.

(a) !primitive random val -> val.

§8. Printing. These print data of various kinds:

(a) primitive !print val -> void. Print text.
(b) primitive !printnumber val -> void. Print a (signed) number in decimal.
(c) primitive !printchar val -> void. Print a character value.
(d) primitive !printnl void -> void. Print a newline. (This is needed because some of our VMs use character 10 for newline, and crash on 13, and others vice versa.)
(e) primitive !printdword val -> void. Print a dictionary word.
(f) primitive !printstring val -> void. Print a packed string.

There are also two primitive ways to change the visual style of text:

(a) primitive !font val -> void. Change to fixed-width font if value is 1, or regular if 0.
(b) primitive !style val -> void. Change to this text style.

The effect of these will depend on the platform the final Inter code is generated for. If the value supplied to !style is 0, 1, 2 or 3, then this should make an effort to achieve roman, bold, italic, or reverse-video type, respectively, and that should apply across all platforms. Use of any other value is likely to be less portable. On C, for example, all other uses of !style are (Inform) text values which supply names for styles.

Lastly, a primitive for a rum feature of Inform 6 allowing for the display of "box quotations" on screen:

(a) primitive !box val -> void.

§9. Stack access. The stack is not directly accessible anywhere in memory, so the only access is via the following.

(a) primitive !push val -> void. Push value onto the stack.
(b) primitive !pull ref -> void. Pull value from the stack and write it into the storage referred to. Values on the stack have unchecked kinds: it's up to the author not to pull an inappropriate value.

§10. Accessing storage. Here the ref term is a refernce to a piece of storage: a property of an instance, or a global variable, or an entry in memory, for example.

(a) primitive !store ref val -> val. Put the value in ref.
(b) primitive !setbit ref val -> void. Set bits in the mask val in ref.
(c) primitive !clearbit ref val -> void. Clear bits in the mask val in ref.
(d) primitive !postincrement ref -> val. Performs the equivalent of ref++.
(e) primitive !preincrement ref -> val. Performs the equivalent of ++ref.
(f) primitive !postdecrement ref -> val. Performs the equivalent of ref--.
(g) primitive !predecrement ref -> val. Performs the equivalent of --ref.

Memory can be accessed with the following. The first value is the address of the array; the second is an offset, that is, with 0 being the first entry, 1 the second, and so on. "Word" in this context means either an int16 or an int32, depending on what virtual machine are compiling to.

(a) primitive !lookup val val -> val. Find word at this word offset.
(b) primitive !lookupbyte val val -> val. Find byte at this byte offset.

Properties, like memory, can be converted to ref in order to write to them, and are accessible with propertyvalue. Their existence can be tested with propertyexists; the other two opcodes here are for the handful of "inline property values", where a property stores not a single value but a small array. In each of the four ternary property primitives, the operands are K, the weak kind ID of the owner; O, the owner; and P, the property. For properties of objects, K will always be OBJECT_TY.

propertyarray and propertylength both produce 0 (but not a run-time error) if called on a property value which does not exist, or is not an inline array. In particular, they always produce 0 if the owner O is not an object, since only objects can have inline property values.

(a) primitive !propertyvalue val val val -> val.
(b) primitive !propertyarray val val val -> val.
(c) primitive !propertylength val val val -> val.
(d) primitive !propertyexists val val val -> val.

§11. Indirect function calls. Invocations of functions can only be made with inv when the function is specified as a constant, and when its signature is therefore known. If we need to call "whatever function this variable refers to", we have to use one of the following. They differ only in their signatures. The first value is the function address, and subsequent ones are arguments.

(a) primitive !indirect0v val -> void.
(b) primitive !indirect1v val val -> void.
(c) primitive !indirect2v val val val -> void.
(d) primitive !indirect3v val val val val -> void.
(e) primitive !indirect4v val val val val val -> void.
(f) primitive !indirect5v val val val val val val -> void.
(g) primitive !indirect0 val -> val.
(h) primitive !indirect1 val val -> val.
(i) primitive !indirect2 val val val -> val.
(j) primitive !indirect3 val val val val -> val.
(k) primitive !indirect4 val val val val val -> val.
(l) primitive !indirect5 val val val val val val -> val.

§12. External function calls. The following calls a function which is not part of the program itself, and which is assumed to be provided by code written in a different programming language. It cannot be used when Inter is being generated to Inform 6 code, because I6 has no ability to link with external code; but it can be used when generating C, for example.

The first value must be a literal double-quoted text, and is the name of the external function. The second value is an argument to pass to it; and the result is whatever value it returns.

(a) primitive !externalcall val val -> val.

§13. Control flow. The simplest control statement is an "if". Note that a different primitive is used if there is an "else" attached: it would be impossible to use the same primitive for both because they couldn't have the same signature.

!ifdebug is an oddity: it executes the code only if the program is being compiled in "debugging mode". (In Inform, that would mean that the story file is being made inside the application, or else released in a special testing configuration.) While the same effect could be achieved using conditional compliation splats, this is much more efficient.

(a) primitive !if val code -> void.
(b) primitive !ifelse val code code -> void.
(c) primitive !ifdebug code -> void.

There are then several loops.

(a) primitive !while val code -> void. Similar to while in C.
(b) primitive !for val val val code -> void. Similar to for in C.
(c) primitive !objectloopx ref val code -> void. A loop over instances, stored in the variable ref, of the kind of object val.
(d) primitive !objectloop ref val val code -> void. A more general form, where the secomd val is a condition to be evaluated which decides whether to execute the code for given ref value.

A switch statement takes a value, and then executes at most one of an arbitrary number of possible code segments. This can't be implemented with a single primitive, because its signature would have to be of varying lengths with different uses (since some switches have many cases, some few). Instead: a switch takes a single code, but that code can in turn contain only invocations of !case, followed optionally by one of !default.

(a) primitive !switch val code -> void.
(b) primitive !case val code -> void.
(c) primitive !default code -> void.

This looks a little baroque, but it works in practice:

    inv !switch
        val K_number X
        code
            inv !case
                val K_number 1
                code
                    inv !print
                        val K_text "One!"
            inv !case
                val K_number 2
                code
                    inv !print
                        val K_text "Two!"
            inv !default
                code
                    inv !print
                        val K_text "Something else!"

As in most C-like languages, there are primitives for:

(a) primitive !break void -> void. Exit the innermost switch case or loop.
(b) primitive !continue void -> void. Complete the current iteration of the innermost loop.

Two ways to terminate what's happening:

(a) primitive !return val -> void. Finish the current function, giving the supplied value as the result if the function is being executed in a value context, and throwing it away if not.
(b) primitive !quit void -> void. Halt the whole program immediately.

And, lastly, the lowest-level way to travel:

(a) primitive !jump lab -> void. Jumo to this label in the current function.

§14. Interactive fiction-only primitives. The following would make no sense in a general-purpose program. Most mirror very low-level I6 features. First, the spatial containment object tree:

(a) primitive !move val val -> void. Moves first to second (both are objects).
(b) primitive !in val val -> val. Tests if first is in second (both are objects).
(c) primitive !notin val val -> val. Negation of same.
(d) primitive !child val -> val. Finds the child node of an object.
(e) primitive !children val -> val. The number of children: which may be 0.
(f) primitive !parent val -> val. Finds the parent of an object.
(g) primitive !sibling val -> val. Finds the sibling of an object.

Object class membership:

(a) primitive !ofclass val val -> val. Does the first belong to the enumerated subkind whose weak type ID is the second value?
(b) primitive !metaclass val -> val. Returns Class, Object, Routine, String or 0 depending on the value supplied: see the Inform 6 Designer's Manual for more on this.