** 5.5 Language Elements Seen So Far Chapters 4 and 5 have covered Scala’s language elements to express expressions and types comprising of primitive data and functions. The context-free syntax of these language elements is given below in extended Backus-Naur form, where ‘|’ denotes alternatives, [...] denotes option (0 or 1 occurrence), and {...} denotes repetition (0 or more occurrences). *** Characters: Scala programs are sequences of (Unicode) characters. We distinguish the following character sets: - whitespace, such as ‘’, tabulator, or newline characters, - letters ‘a’ to ‘z’, ‘A’ to ‘Z’, - digits ‘0’ to ‘9’, - the delimiter characters . , ; ( ) { } [ ] \ " ’ - operator characters, such as ‘#’ ‘+’, ‘:’. Essentially, these are printable characters which are in none of the character sets above. *** Lexemes: ident = letter {letter | digit} | operator { operator } | ident ’_’ ident literal = “as in Java” Literals are as in Java. They define numbers, characters, strings, or boolean values. Examples of literals as 0, 1.0e10, ’x’, "he said "hi!"", or true. Identifiers can be of two forms. They either start with a letter, which is followed by a (possibly empty) sequence of letters or symbols, or they start with an operator character, which is followed by a (possibly empty) sequence of operator characters. Both forms of identifiers may contain underscore characters ‘_’. Furthermore, an underscore character may be followed by either sort of identifier. Hence, the following are all legal identifiers: x Room10a + -- foldl_: +_vector It follows from this rule that subsequent operator-identifiers need to be separated by whitespace. For instance, the input x+-y is parsed as the three token sequence x, +-, y. If we want to express the sum of x with the negated value of y, we need to add at least one space, e.g. x+ -y. The $ character is reserved for compiler-generated identifiers; it should not be used in source programs. The following are reserved words, they may not be used as identifiers: abstract case catch class def do else extends false final finally for if implicit import match new null object override package private protected requires return sealed super this throw trait try true type val var while with yield _ : = => <- <: <% >: # @ *** Types: Type = SimpleType | FunctionType FunctionType = SimpleType ’=>’ Type | ’(’ [Types] ’)’ ’=>’ Type SimpleType = Byte | Short | Char | Int | Long | Float | Double | Boolean | Unit | String Types = Type {‘,’ Type} Types can be: - number types Byte, Short, Char, Int, Long, Float and Double (these are as in Java), - the type Boolean with values true and false, - the type Unit with the only value (), - the type String, - function types such as (Int, Int) => Int or String => Int => String. *** Expressions: Expr = InfixExpr | FunctionExpr | if ’(’ Expr ’)’ Expr else Expr InfixExpr = PrefixExpr | InfixExpr Operator InfixExpr Operator = ident PrefixExpr = [’+’ | ’-’ | ’!’ | ’~’ ] SimpleExpr SimpleExpr = ident | literal | SimpleExpr ’.’ ident | Block FunctionExpr = (Bindings | Id) ’=>’ Expr Bindings = ‘(’ Binding {‘,’ Binding} ‘)’ Binding = ident [’:’ Type] Block = ’{’ {Def ’;’} Expr ’}’ Expressions can be: - identifiers such as x, isGoodEnough, *, or +-, - literals, such as 0, 1.0, or "abc", - field and method selections, such as System.out.println, - function applications, such as sqrt(x), - operator applications, such as -x or y + x, - conditionals, such as if (x < 0) -x else x, - blocks, such as { val x = abs(y) ; x * 2 }, - anonymous functions, such as x => x + 1 or (x: Int, y: Int) => x + y. *** Definitions: Def = FunDef | ValDef FunDef = ’def’ ident {’(’ [Parameters] ’)’} [’:’ Type] ’=’ Expr ValDef = ’val’ ident [’:’ Type] ’=’ Expr Parameters = Parameter {’,’ Parameter} Parameter = ident ’:’ [’=>’] Type Definitions can be: - function definitions such as def square(x: Int): Int = x * x, - value definitions such as val y = square(2).
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