# package token

`import "go/token"`

Package token defines constants representing the lexical tokens of the Go programming language and basic operations on tokens (printing, predicates).

position.go serialize.go token.go

## Constants

A set of constants for precedence-based expression parsing. Non-operators have lowest precedence, followed by operators starting with precedence 1 up to unary operators. The highest precedence serves as "catch-all" precedence for selector, indexing, and other operator and delimiter tokens.

## type File

A File is a handle for a file belonging to a FileSet. A File has a name, size, and line offset table.

### func (*File) AddLine

AddLine adds the line offset for a new line. The line offset must be larger than the offset for the previous line and smaller than the file size; otherwise the line offset is ignored.

### func (*File) AddLineInfo

AddLineInfo adds alternative file and line number information for a given file offset. The offset must be larger than the offset for the previously added alternative line info and smaller than the file size; otherwise the information is ignored.

AddLineInfo is typically used to register alternative position information for //line filename:line comments in source files.

### func (*File) Base

Base returns the base offset of file f as registered with AddFile.

### func (*File) Line

Line returns the line number for the given file position p; p must be a Pos value in that file or NoPos.

### func (*File) LineCount

LineCount returns the number of lines in file f.

### func (*File) MergeLine

MergeLine merges a line with the following line. It is akin to replacing the newline character at the end of the line with a space (to not change the remaining offsets). To obtain the line number, consult e.g. Position.Line. MergeLine will panic if given an invalid line number.

### func (*File) Name

Name returns the file name of file f as registered with AddFile.

### func (*File) Offset

Offset returns the offset for the given file position p; p must be a valid Pos value in that file. f.Offset(f.Pos(offset)) == offset.

### func (*File) Pos

Pos returns the Pos value for the given file offset; the offset must be <= f.Size(). f.Pos(f.Offset(p)) == p.

### func (*File) Position

Position returns the Position value for the given file position p. Calling f.Position(p) is equivalent to calling f.PositionFor(p, true).

### func (*File) PositionFor

PositionFor returns the Position value for the given file position p. If adjusted is set, the position may be adjusted by position-altering //line comments; otherwise those comments are ignored. p must be a Pos value in f or NoPos.

### func (*File) SetLines

SetLines sets the line offsets for a file and reports whether it succeeded. The line offsets are the offsets of the first character of each line; for instance for the content "ab\nc\n" the line offsets are {0, 3}. An empty file has an empty line offset table. Each line offset must be larger than the offset for the previous line and smaller than the file size; otherwise SetLines fails and returns false. Callers must not mutate the provided slice after SetLines returns.

### func (*File) SetLinesForContent

SetLinesForContent sets the line offsets for the given file content. It ignores position-altering //line comments.

### func (*File) Size

Size returns the size of file f as registered with AddFile.

## type FileSet

A FileSet represents a set of source files. Methods of file sets are synchronized; multiple goroutines may invoke them concurrently.

### func NewFileSet

NewFileSet creates a new file set.

### func (*FileSet) AddFile

AddFile adds a new file with a given filename, base offset, and file size to the file set s and returns the file. Multiple files may have the same name. The base offset must not be smaller than the FileSet's Base(), and size must not be negative. As a special case, if a negative base is provided, the current value of the FileSet's Base() is used instead.

Adding the file will set the file set's Base() value to base + size + 1 as the minimum base value for the next file. The following relationship exists between a Pos value p for a given file offset offs:

    int(p) = base + offs

with offs in the range [0, size] and thus p in the range [base, base+size]. For convenience, File.Pos may be used to create file-specific position values from a file offset.

### func (*FileSet) Base

Base returns the minimum base offset that must be provided to AddFile when adding the next file.

### func (*FileSet) File

File returns the file that contains the position p. If no such file is found (for instance for p == NoPos), the result is nil.

### func (*FileSet) Iterate

Iterate calls f for the files in the file set in the order they were added until f returns false.

### func (*FileSet) Position

Position converts a Pos p in the fileset into a Position value. Calling s.Position(p) is equivalent to calling s.PositionFor(p, true).

### func (*FileSet) PositionFor

PositionFor converts a Pos p in the fileset into a Position value. If adjusted is set, the position may be adjusted by position-altering //line comments; otherwise those comments are ignored. p must be a Pos value in s or NoPos.

### func (*FileSet) Read

Read calls decode to deserialize a file set into s; s must not be nil.

### func (*FileSet) Write

Write calls encode to serialize the file set s.

## type Pos

Pos is a compact encoding of a source position within a file set. It can be converted into a Position for a more convenient, but much larger, representation.

The Pos value for a given file is a number in the range [base, base+size], where base and size are specified when adding the file to the file set via AddFile.

To create the Pos value for a specific source offset (measured in bytes), first add the respective file to the current file set using FileSet.AddFile and then call File.Pos(offset) for that file. Given a Pos value p for a specific file set fset, the corresponding Position value is obtained by calling fset.Position(p).

Pos values can be compared directly with the usual comparison operators: If two Pos values p and q are in the same file, comparing p and q is equivalent to comparing the respective source file offsets. If p and q are in different files, p < q is true if the file implied by p was added to the respective file set before the file implied by q.

The zero value for Pos is NoPos; there is no file and line information associated with it, and NoPos().IsValid() is false. NoPos is always smaller than any other Pos value. The corresponding Position value for NoPos is the zero value for Position.

### func (Pos) IsValid

IsValid reports whether the position is valid.

## type Position

Position describes an arbitrary source position including the file, line, and column location. A Position is valid if the line number is > 0.

### func (*Position) IsValid

IsValid reports whether the position is valid.

### func (Position) String

String returns a string in one of several forms:

    file:line:column    valid position with file name
    line:column         valid position without file name
    file                invalid position with file name
    -                   invalid position without file name

## type Token

Token is the set of lexical tokens of the Go programming language.

The list of tokens.

### func Lookup

Lookup maps an identifier to its keyword token or IDENT (if not a keyword).

### func (Token) IsKeyword

IsKeyword returns true for tokens corresponding to keywords; it returns false otherwise.

### func (Token) IsLiteral

IsLiteral returns true for tokens corresponding to identifiers and basic type literals; it returns false otherwise.

### func (Token) IsOperator

IsOperator returns true for tokens corresponding to operators and delimiters; it returns false otherwise.

### func (Token) Precedence

Precedence returns the operator precedence of the binary operator op. If op is not a binary operator, the result is LowestPrecedence.

### func (Token) String

String returns the string corresponding to the token tok. For operators, delimiters, and keywords the string is the actual token character sequence (e.g., for the token ADD, the string is "+"). For all other tokens the string corresponds to the token constant name (e.g. for the token IDENT, the string is "IDENT").