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Expressions.md

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Expressions and Operators

TODO

Literals

Much like C or most other programming languages, ZScript has object literals, including string literals, integer literals, float literals, name literals, boolean literals, and the null pointer.

String literals

String literals take the same form as in C:

"text here"

String literals have character escapes, which are formed with a backslash and a character. Character escapes include:

Spelling Output
\" A literal ".
\\ A literal \.
\ followed by newline Concatenates the next line with this one.
\a Byte 0x07 (BEL - bell, anachronism.)
\b Byte 0x08 (BS - backspace, anachronism.)
\c Byte 0x1c (TEXTCOLOR_ESCAPE.)
\f Byte 0x0c (FF - form feed, anachronism.)
\n Byte 0x0a (LF - new line.)
\t Byte 0x09 (HT - tab.)
\r Byte 0x0d (CR - return.)
\v Byte 0x0b (VT - vertical tab, anachronism.)
\? A literal ? (obsolete anachronism.)
\xnn or \Xnn Byte 0xnn.
\nnn Byte 0nnn (octal.)

To quote cppreference, "of the octal escape sequences, \0 is the most useful because it represents the terminating null character in null-terminated strings."

String literals, also like C and C++, will be concatenated when put directly next to each other. For example, this:

"text 1" "text 2"

Will be parsed as a single string literal with the text "text 1text 2".

Class type literals

Class type literals take the same form as string literals, but do note that they are not the same.

Name literals

Name literals are similar to string literals, though they use apostrophes instead of quote marks:

'text here'

They do not concatenate like string literals, and do not have character escapes.

Integer literals

Integer literals are formed similarly to C. They may take one of three forms, and be typed uint or int based on whether there is a u or U at the end or not.

The parser also supports an optional l/L suffix as in C, though it does not actually do anything, and it is advised you do not use it for potential forward compatibility purposes.

Integer literals can be in the basic base-10/decimal form:

1234567890 // int
500u       // uint

Base-16/hexadecimal form, which may use upper- or lower-case decimals and 0x prefix, depending on user preference:

0x123456789ABCDEF0
0XaBcDeF0 // don't do this, please.
0x7fff
0x7FFFFFFF

And, base-8/octal form, prefixed with a 0:

0777
0414444

Float literals

Float literals, much like integer literals, are formed similarly to C, but they do not support hex-float notation. Float literals support exponent notation.

The parser supports an optional f/F suffix as in C, though it does not actually do anything, and it is advised you do not use it for potential forward compatibility purposes.

Float literals can be formed in a few ways:

0.5 //=> 0.5
.5  //=> 0.5
1.  //=> 1.0

And with exponents:

0.5e+2 //=> 50
50e-2  //=> 0.5

Boolean literals

The two boolean literals are spelled false and true, and much like C, can decay to the integer literals 0 and 1.

Null pointer

The null pointer literal is spelled null and represents an object that does not exist in memory. Like C, it is not equivalent to the integer literal 0, and is more similar to C++'s nullptr.

Expressions

Expressions contain literals or other such expressions of objects, including arithmetic and various conditions.

Primary expressions

Basic expressions, also known as primary expressions, can be one of:

  • An identifier for a constant or variable.
  • The Super keyword.
  • Any object literal.
  • A vector literal.
  • An expression in parentheses.

Identifiers work as you expect, they reference a variable or constant. The Super keyword references the parent type or any member within it.

Vector literals

Vector literals are not under object literals as they are not constants in the same manner as other literals, since they contain expressions within them. As such, they are expressions, not proper value-based literals. They can be formed with:

( X , Y )     //=> vector2, where X is not a vector2
( X , Y )     //=> vector3, where X *is* a vector2
( X , Y , Z ) //=> vector3

All components must have type double, except in the second grammar where X is vector2.

Postfix expressions

Postfix expressions are affixed at the end of an expression, and are used for a large variety of things, although the actual amount of postfix expressions is small:

Form Description
A ( $[Argument-list]$ ) Function call.
Type ( A ) Type cast.
( class < Type > ) ( A ) Class type reference cast.
A [ B ] Array access.
A.B Member access.
A++ Post-increment. This increments (adds 1 to) the object after the expression is evaluated.
A-- Post-decrement. This decrements (subtracts 1 from) the object after the expression is evaluated.

Argument list

The syntax for an argument list is:

Expression $[ , Expression]$...

Function calls may name arguments which have defaults with the syntax Identifier : Expression, possibly skipping over other defaulted arguments. After the first named defaultable argument, all other arguments must be named as well.

Unary expressions

Unary expressions are affixed at the beginning of an expression. The simplest example of a unary expression is the negation operator, -, as in -500. Unary expressions include:

Form Description
- A Negation.
! A Logical negation, "not."
++ A Pre-increment. This adds 1 to the object and evaluates as the resulting value.
-- A Pre-decrement. This subtracts 1 from the object and evaluates as the resulting value.
~ A Bitwise negation. Flips all bits in an integer.
+ A Affirmation. Does not actually do anything.
alignof A Evaluates the alignment of the type of an expression. Unknown purpose.
sizeof A Evaluates the size of the type of an expression. Unknown purpose.

Binary expressions

Binary expressions operate on two expressions, and are the most common kind of expression. They are used inline like regular math syntax, i.e. 1 + 1. Binary expressions include:

Form Description
A + B Addition.
A - B Subtraction.
A * B Multiplication.
A / B Division quotient.
A % B Division remainder, also known as "modulus." Unlike C, this works on floats, too.
A ** B Exponent ("power of.")
A << B Left bitwise shift.
A >> B Right bitwise shift.
A >>> B Right unsigned bitwise shift.
A cross B Vector cross-product.
A dot B Vector dot-product.
A .. B Concatenation, creates a string from two values.
A < B true if A is less than B.
A > B true if A is greater than B.
A <= B true if A is less than or equal to B.
A >= B true if A is greater than or equal to B.
A == B true if A is equal to B.
A != B true if A is not equal to B.
A ~== B true if A is approximately equal to B. For strings this is a case-insensitive comparison, for floats and vectors this checks if the difference between the two is smaller than ε.
A && B true if A and B are both true.
A || B true if A or B is true.
A is "B" true if A's type is equal to or a descendant of B.
A <>= B Signed difference between A and B.
A & B Bitwise AND.
A ^ B Bitwise XOR.
A | B Bitwise OR.
A :: B Scope operator. Not implemented yet.

Assignment expressions

Assignment expressions are a subset of binary expressions which are never constant expressions. They assign a value to another value, as one might guess.

Form Description
A = B Assigns B to A.
A += B Assigns A + B to A.
A -= B Assigns A - B to A.
A *= B Assigns A * B to A.
A /= B Assigns A / B to A.
A %= B Assigns A % B to A.
A <<= B Assigns A << B to A.
A >>= B Assigns A >> B to A.
A >>>= B Assigns A >>> B to A.
A |= B Assigns A | B to A.
A &= B Assigns A & B to A.
A ^= B Assigns A ^ B to A.

Ternary expression

The ternary expression is formed A ? B : C, and will evaluate to B if A is true, or C if it is false.