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millfork/doc/lang/syntax.md
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Syntax

For information about types, see Types.
For information about literals, see Literals.
For information about assembly, see Using assembly within Millfork programs.

Comments

Comments start with // and last until the end of line.

Declarations

Variable declarations

A variable declaration can happen at either top level of a file (global variables), or a top level of a function (local variables).

Syntax:

[<storage>] <type> <name> [@<address>] [= <initial_value>]

  • <storage> can be only specified for local variables. It can be either stack, static, register or nothing. register is only a hint for the optimizer. See the description of variable storage.

  • <address> is a constant expression that defines where in the memory the variable will be located. If not specified, it will be located according to the usual allocation rules. stack variables cannot have a defined address.

  • <initial_value> is a constant expression that contains the initial value of the variable. Only global variables can be initialized that way. The behaviour is undefined when targeting a ROM-based platform.

Constant declarations

const <type> <name> = <value>

TODO

Array declarations

An array is a continuous sequence of bytes in memory.

Syntax:

array <name> [[<size>]] [@<address>] [= <initial_values>]

TODO

Function declarations

A function can be declared at the top level. For more details, see Functions

import statements

TODO

Statements

Expression statement

TODO

if statement

Syntax:

if <expression> {
    <body>
}

if <expression> {
    <body>
} else {
    <body>
}

return statement

Syntax:

return

return <expression>

return[] statement (return dispatch)

Syntax examples:

return [a + b] {
   0   @ underflow
   255 @ overflow
   default @ nothing
}

return [getF()] {
   1 @ function1
   2 @ function2
   default(5) @ functionDefault
}

return [i] (param1, param2) {
   1,5,8 @ function1(4, 6)
   2     @ function2(9)
   default(0,20) @ functionDefault
}

Return dispatch calculates the value of an index, picks the correct branch, assigns some global variables and jumps to another function.

The index has to evaluate to a byte. The functions cannot be macro and shouldn't have parameters. Jumping to a function with parameters gives those parameters undefined values.

The functions are not called, so they don't return to the function the return dispatch statement is in, but to its caller. The return values are passed along. If the dispatching function has a non-void return type different that the type of the function dispatched to, the return value is undefined.

If the default branch exists, then it is used for every missing index value between other supported values. Optional parameters to default specify the maximum, or both the minimum and maximum supported index value. In the above examples: the first example supports values 0–255, second 1–5, and third 0–20.

If the index has an unsupported value, the behaviour is formally undefined, but in practice the program will simply crash.

Before jumping to the function, the chosen global variables will be assigned parameter values. Variables have to be global byte-sized. Some simple array indexing expressions are also allowed. Parameter values have to be constants. For example, in the third example one of the following will happen:

  • if i is 1, 5 or 8, then param1 is assigned 4, param2 is assigned 6 and then function1 is called;

  • if i is 2, then param1 is assigned 9, param2 is assigned an undefined value and then function2 is called;

  • if i is any other value from 0 to 20, then param1 and param2 are assigned undefined values and then functionDefault is called;

  • if i has any other value, then undefined behaviour.

while and do-while statements

Syntax:

while <expression> {
    <body>
}

do {
    <body>
} while <expression>

for statements

Warning: for loops are a bit buggy.

Syntax:

for <variable>,<start>,<direction>,<end> {
}

  • <variable> – an already defined numeric variable

  • <direction> – the range to traverse:

    • to – from <start> inclusive to <end> inclusive, in ascending order (e.g. 0,to,9 to traverse 0, 1,... 9)

    • downto – from <start> inclusive to <end> inclusive, in descending order (e.g. 9,downto,0 to traverse 9, 8,... 0)

    • until – from <start> inclusive to <end> exclusive, in ascending order (e.g. 0,until,10 to traverse 0, 1,... 9)

    • parallelto – the same as to, but the iterations may be executed in any order

    • paralleluntil – the same as until, but the iterations may be executed in any order

    There is no paralleldownto, because it would do the same as parallelto.

break and continue statements

Syntax:

break
break for
break while
break do
break <variable>
continue
continue for
continue while
continue do
continue <variable>

asm statements

See Using assembly within Millfork programs.