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310 lines
8.4 KiB
Markdown
310 lines
8.4 KiB
Markdown
[< back to index](../index.md)
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# Syntax
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For information about types, see [Types](./types.md).
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For information about literals, see [Literals](./literals.md).
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For information about assembly, see [Using assembly within Millfork programs](./assembly.md).
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## Comments
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Comments start with `//` and last until the end of line.
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## Declarations
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### Variable declarations
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A variable declaration can happen at either top level of a file (*global* variables),
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or a top level of a function (*local* variables).
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Syntax:
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`[segment(<segment>)] [volatile] [<storage>] <type> <name> [@<address>] [= <initial_value>]`
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* `<segment>`: segment name; if absent, then defaults to `default`.
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* `volatile` means that the variable is volatile.
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The optimizer shouldn't remove or reorder accesses to volatile variables.
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Volatile variables cannot be declared as `register` or `stack.
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* `<storage>` can be only specified for local variables. It can be either `stack`, `static`, `register` or nothing.
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`register` is only a hint for the optimizer.
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See [the description of variable storage](../abi/variable-storage.md).
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* `<address>` is a constant expression that defines where in the memory the variable will be located.
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If not specified, it will be located according to the usual allocation rules.
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`stack` variables cannot have a defined address.
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* `<initial_value>` is a constant expression that contains the initial value of the variable.
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Only global variables can be initialized that way.
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The behaviour is undefined when targeting a ROM-based platform.
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For every variable `x` larger than a byte, extra subvariables are defined:
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* if `x` is of type `word` or `pointer`:
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* constituent bytes, from low to high: `x.lo`, `x.hi`
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* if `x` is of type `farword`:
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* constituent bytes, from low to high: `x.b0`, `x.b1`, `x.b2`
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* partial words: `x.loword` (=`x.b1:x.b0`), `x.hiword` (=`x.b2:x.b1`)
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* if `x` is of type `long`:
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* constituent bytes, from low to high: `x.b0`, `x.b1`, `x.b2`, `x.b3`
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* partial words: `x.loword` (=`x.b1:x.b0`), `x.hiword` (=`x.b3:x.b2`)
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### Constant declarations
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`const <type> <name> = <value>`
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TODO
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### Alias definitions
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`alias <alias> = <name> [!]`
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Sets an alias for a global name.
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Unless shadowed by a local name, the alias will point to the given global object:
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byte x
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alias a = x
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void f() {
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a = 5 // writes to the global variable x
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}
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void f() {
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byte a
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a = 5 // writes to the local variable a
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}
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Aliases can be used for variables, arrays, constants, functions, and types,
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but not for text encodings, array formats or keywords.
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If the alias definition is followed by a `!`, then the alias overrides any other definition of that name.
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This allows for overriding definitions of library functions by another library:
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void f() {}
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void g() {}
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alias f = g!
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// now the original f is removed and all calls to f will call g instead
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### Array declarations
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An array is a continuous sequence of bytes in memory.
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Syntax:
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`[segment(<segment>)] array <name> [[<size>]] [align ( <alignment> )] [@<address>] [= <initial_values>]`
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* `<segment>`: segment name; if absent,
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then defaults to `default_code_segment` as defined for the platform if the array has initial values,
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or to `default` if it doesn't.
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* `<size>`: either a constant number, which then defines the size of the array,
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or a name of a plain enumeration type, in which case changes the type of the index to that enumeration
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and declares the array size to be equal to the number of variants in that enumeration.
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If the size is not specified here, then it's deduced from the `<initial_values>`.
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If the declared size and the size deduced from the `<initial_values>` don't match, then an error is raised.
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* `<alignment>` is either a numeric literal that is a power of 2, or keyword `fast`.
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The array will be allocated at the address divisible by alignment.
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`fast` means different things depending on the target platform:
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* on 6502, it means that the array will not cross a page boundary
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* on Z80, it means that the array will not cross a page boundary
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TODO
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### Function declarations
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A function can be declared at the top level. For more details, see [Functions](./functions.md)
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## `import` statements
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TODO
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## Statements
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### Expression statement
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TODO
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### `if` statement
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Syntax:
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```
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if <expression> {
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<body>
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}
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```
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```
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if <expression> {
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<body>
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} else {
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<body>
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}
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```
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```
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if <expression> {
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<body>
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} else if <expression> {
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<body>
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} else {
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<body>
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}
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```
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### `return` statement
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Syntax:
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```
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return
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```
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```
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return <expression>
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```
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### `return[]` statement (return dispatch)
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Syntax examples:
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```
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return [a + b] {
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0 @ underflow
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255 @ overflow
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default @ nothing
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}
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```
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```
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return [getF()] {
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1 @ function1
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2 @ function2
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default(5) @ functionDefault
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}
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```
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```
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return [i] (param1, param2) {
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1,5,8 @ function1(4, 6)
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2 @ function2(9)
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default(0,20) @ functionDefault
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}
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```
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Return dispatch calculates the value of an index, picks the correct branch,
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assigns some global variables and jumps to another function.
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The index has to evaluate to a byte or to an enum. The functions cannot be `macro` and shouldn't have parameters.
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Jumping to a function with parameters gives those parameters undefined values.
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The functions are not called, so they don't return to the function the return dispatch statement is in, but to its caller.
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The return values are passed along. If the dispatching function has a non-`void` return type different that the type
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of the function dispatched to, the return value is undefined.
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If the `default` branch exists, then it is used for every missing index.
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If the index type is an non-empty enum, then the default branch supports all the other values.
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Otherwise, the `default` branch handles only the missing values between other supported values.
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In this case, you can override it with optional parameters to `default`.
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They specify the maximum, or both the minimum and maximum supported index value.
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In the above examples: the first example supports values 0–255, second 1–5, and third 0–20.
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If the index has an unsupported value, the behaviour is formally undefined, but in practice the program will simply crash.
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Before jumping to the function, the chosen global variables will be assigned parameter values.
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Variables have to be global byte-sized. Some simple array indexing expressions are also allowed.
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Parameter values have to be constants.
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For example, in the third example one of the following will happen:
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* if `i` is 1, 5 or 8, then `param1` is assigned 4, `param2` is assigned 6 and then `function1` is called;
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* if `i` is 2, then `param1` is assigned 9, `param2` is assigned an undefined value and then `function2` is called;
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* if `i` is any other value from 0 to 20, then `param1` and `param2` are assigned undefined values and then `functionDefault` is called;
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* if `i` has any other value, then undefined behaviour.
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### `while` and `do-while` statements
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Syntax:
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```
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while <expression> {
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<body>
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}
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```
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```
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do {
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<body>
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} while <expression>
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```
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### `for` statements
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**Warning: `for` loops are a bit buggy.**
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Syntax:
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```
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for <variable> , <start> , <direction> , <end> {
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}
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for <variable> : <enum type> {
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}
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for <variable> : [ <comma separated expressions> ] {
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}
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```
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* `<variable>` – an already defined numeric variable
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* `<direction>` – the range to traverse:
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* `to` – from `<start>` inclusive to `<end>` inclusive, in ascending order
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(e.g. `0,to,9` to traverse 0, 1,... 9)
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* `downto` – from `<start>` inclusive to `<end>` inclusive, in descending order
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(e.g. `9,downto,0` to traverse 9, 8,... 0)
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* `until` – from `<start>` inclusive to `<end>` exclusive, in ascending order
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(e.g. `0,until,10` to traverse 0, 1,... 9)
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* `parallelto` – the same as `to`, but the iterations may be executed in any order
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* `paralleluntil` – the same as `until`, but the iterations may be executed in any order
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There is no `paralleldownto`, because it would do the same as `parallelto`.
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* `<enum type>` – traverse enum constants of given type, in arbitrary order
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* `<comma separated expressions>` – traverse every value in the list
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### `break` and `continue` statements
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Syntax:
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```
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break
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break for
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break while
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break do
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break <variable>
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continue
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continue for
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continue while
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continue do
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continue <variable>
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```
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### `asm` statements
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See [Using assembly within Millfork programs](./assembly.md).
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