Exceptions

Exceptions typically indicate unexpected situations occuring during program execution. Exceptions are caught with the try statement (See See Try Statement) and generated with the Throw built-in function. Processing exceptions require knowledged about the format of exception objects, in particularly the type of the exception, which allows you to distinguish between the possible situations that occurred.

See Exceptions thrown by the built-in functions lists the exceptions thrown by the built-in WebL functions. By convention, the exception type (eg ArgumentError etc.) is indicated by the type field of the exception object. Also by convention, the msg field of the exception object gives information on why the exception occured.

Operators and statements can also generate exceptions, as explained in the following paragraphs.

All operators will throw an OperandMismatch exception in case the operands to the operator are not of the expected value type.

Function or method application (eg calling a function or method) can throw the following exceptions:

NoSuchField - Object does not have such field.

NotAFunctionOrMethod - Left-hand side is not callable.

NotAnObject - Left hand side is not an object.

ArgumentError - Number of actual and formal arguments do not match.

Variable assigment with "=" can throw the following exceptions:

FieldError - Unknown field or illegal field assignment.

NotAnObject - Left hand side is not an object field.

NotAVariable - Left hand side is not a variable.

Field definition with ":=" can throw the following exceptions:

FieldDefinitionError - Could not define field.

NotAnObject - Left hand side is not an object.

Indexing into a type with "[ ]" or "." can throw the following exceptions:

IndexRangeError - Index is out of range.

ArgumentError - Index is not of the expected type.

NoSuchField - Object does not have such field.

NotAnObject - Left hand side is not an object.

The if, repeat, while and catch statements will throw a GuardError exception if the guard expression does not return a boolean value type.

The every statement will throw a NotEnumerable exception if the object does not have enumerable contents.

The lock statement will throw a NotAnObject exception if an attempt is made to lock on a non-object value type.

Exceptions thrown by the built-in functions
Function	Exceptions
Assert(x: bool): nil	ArgumentError - Incorrect or wrong number of arguments AssertFailed - Assertion failed
BeginTag(q: piece): tag	ArgumentError - Incorrect or wrong number of arguments
Boolp(x): bool	ArgumentError - Incorrect or wrong number of arguments
Exec(cmd: string): int	ArgumentError - Incorrect or wrong number of arguments
Charp(x): bool	ArgumentError - Incorrect or wrong number of arguments
Children(q: piece): pieceset	ArgumentError - Incorrect or wrong number of arguments
Clone(o: object, p: object, ...): object	ArgumentError - Incorrect or wrong number of arguments
Content(p: page): piece Content(q: piece): piece	NoContent - Page or piece has no content ArgumentError - Incorrect or wrong number of arguments
Delete(s: pieceset): nil Delete(q: piece): nil	ArgumentError - Incorrect or wrong number of arguments
Elem(p: page): pieceset Elem(p: page, name: string): pieceset Elem(q: piece): pieceset Elem(q: piece, name: string): pieceset	ArgumentError - Incorrect or wrong number of arguments
EndTag(q: piece): tag	ArgumentError - Incorrect or wrong number of arguments
Error(x, y, z, ...): nil	No exceptions are thrown
ErrorLn(x, y, z, ...): nil	No exceptions are thrown
Eval(s: string): any	ArgumentError - Incorrect or wrong number of arguments SyntaxError - Cannot evaluate due to syntax error in argument IOException - An IO exception occurred during function execution ReturnException - A return statement was executed outside of a function or method while executing the argument
Exec(cmd: string): int	ArgumentError - Incorrect or wrong number of arguments
Exit(errorcode: int): nil	ArgumentError - Incorrect or wrong number of arguments
ExpandCharEntities(p: page, s: string): string ExpandCharEntities(s: string): string	ArgumentError - Incorrect or wrong number of arguments IOException - An IO exception occurred during function execution
First(l: list): any	ArgumentError - Incorrect or wrong number of arguments EmptyList - Cannot apply first to an empty list
Flatten(s: pieceset): pieceset	ArgumentError - Incorrect or wrong number of arguments
Funp(x): bool	ArgumentError - Incorrect or wrong number of arguments
GC(): nil	ArgumentError - Incorrect or wrong number of arguments
GetURL(url: string): page) GetURL(url: string, params: {object, string}): page) GetURL(url: string, params: {object, string}, headers: object): page) GetURL(url: string, params: {object, string}, headers: object, options: object): page)	ArgumentError - Incorrect or wrong number of arguments NetException - Fetch failed, "statuscode" field of the exception object indicates the reason
GetURL(url: string): page) GetURL(url: string, params: {object, string}): page) GetURL(url: string, params: {object, string}, headers: object): page)	ArgumentError - Incorrect or wrong number of arguments NetException - Fetch failed, "statuscode" field of the exception object indicates the reason
InsertAfter(t: tag; q: piece): nil InsertAfter(t: tag; s: pieceset): nil	ArgumentError - Incorrect or wrong number of arguments
InsertBefore(t: tag; q: piece): nil InsertBefore(t: tag; s: pieceset): nil	ArgumentError - Incorrect or wrong number of arguments
Intp(x): bool	ArgumentError - Incorrect or wrong number of arguments
Listp(x): bool	ArgumentError - Incorrect or wrong number of arguments
Markup(P: page): string Markup(q: piece): string	ArgumentError - Incorrect or wrong number of arguments
Methp(x): bool	ArgumentError - Incorrect or wrong number of arguments
Name(q: piece): string	ArgumentError - Incorrect or wrong number of arguments
Native(classname: string): fun	ArgumentError - Incorrect or wrong number of arguments NativeCodeImportError - Class instantiation failed, access denied, no such method, or not a sub-class of built-in
NewNamedPiece(name: string, t1: tag, t2: tag): piece NewNamedPiece(name: string, q: piece): piece	ArgumentError - Incorrect or wrong number of arguments NotSamePage - The tag arguments to the function do not belong to the same page
NewPage(s: string, mimetype: string): page	ArgumentError - Incorrect or wrong number of arguments NetException - Fetch failed, "statuscode" field of the exception object indicates the reason
NewPiece(q: piece): piece NewPiece(s: string, mimetype: string): piece NewPiece(t1: tag, t2: tag): piece	ArgumentError - Incorrect or wrong number of arguments NotSamePage - The tag arguments to the function do not belong to the same page
NewPieceSet(p: page): pieceset	ArgumentError - Incorrect or wrong number of arguments NotAPiece - The set argument to NewPieceSet must only contain pieces EmptySet - The set argument to NewPieceSet must only contain pieces belonging to the same page NotSamePage - The set argument to NewPieceSet must only contain pieces belonging to the same page
Objectp(x): bool	ArgumentError - Incorrect or wrong number of arguments
PCData(p: page): pieceset PCData(p: piece): pieceset	ArgumentError - Incorrect or wrong number of arguments
Page(q: piece): page Page(t: tag): page	ArgumentError - Incorrect or wrong number of arguments
Pagep(x): bool	ArgumentError - Incorrect or wrong number of arguments
Para(p: page, paraspec: string): pieceset Para(q: piece, paraspec: string): pieceset	ArgumentError - Incorrect or wrong number of arguments
Parent(q: piece): piece	ArgumentError - Incorrect or wrong number of arguments
Pat(p: page, regexp: string): pieceset Pat(q: piece, regexp: string): pieceset	ArgumentError - Incorrect or wrong number of arguments MalformedPattern - Illegal regular expression passed to Pat function
PieceSetp(x): bool	ArgumentError - Incorrect or wrong number of arguments
Piecep(x): bool	ArgumentError - Incorrect or wrong number of arguments
PostURL(url: string): page) PostURL(url: string, params: {object, string}): page) PostURL(url: string, params: {object, string}, headers: object): page) PostURL(url: string, params: {object, string}, headers: object, options: object): page)	ArgumentError - Incorrect or wrong number of arguments NetException - Fetch failed, "statuscode" field of the exception object indicates the reason IOException - An IO exception occurred during function execution
Pretty(p: page): string Pretty(q: piece): string	ArgumentError - Incorrect or wrong number of arguments
Print(x, y, z, ...): nil	No exceptions are thrown
PrintLn(x, y, z, ...): nil	No exceptions are thrown
ReadLn(): string	ArgumentError - Incorrect or wrong number of arguments IOException - An IO exception occurred during function execution
Realp(x): bool	ArgumentError - Incorrect or wrong number of arguments
Replace(a: pieceset, b: pieceset): nil	ArgumentError - Incorrect or wrong number of arguments
Rest(l: list): list	ArgumentError - Incorrect or wrong number of arguments
Retry(x): any	ArgumentError - Incorrect or wrong number of arguments
Select(s: set: f: fun): set Select(l: list, f: fun): list Select(p: pieceset, f: fun): pieceset Select(s: string, from: int, to: int): string	ArgumentError - Incorrect or wrong number of arguments
Seq(p: page, pattern: string): pieceset Seq(q: piece, pattern: string): pieceset	ArgumentError - Incorrect or wrong number of arguments IndexRangeError - Index into list or string out of bounds FunctionReturnTypeNotBoolean - Function argument to Select did not return a boolean value
Setp(x): bool	ArgumentError - Incorrect or wrong number of arguments
Sign(x: int): int Sign(x: real): int	ArgumentError - Incorrect or wrong number of arguments
Size(s: set):int Size(s: string): int Size(l: list): int	ArgumentError - Incorrect or wrong number of arguments
Sleep(ms: int): nil	ArgumentError - Incorrect or wrong number of arguments Interrupted - Sleep function interrupted
Sort(l: list, f: fun): list	ArgumentError - Incorrect or wrong number of arguments FunctionReturnTypeNotInteger - Function argument to Sort did not return an integer value
Stall()	ArgumentError - Incorrect or wrong number of arguments
Stringp(x): bool	ArgumentError - Incorrect or wrong number of arguments
Tagp(x): bool	ArgumentError - Incorrect or wrong number of arguments
Text(p: page): string Text(q: piece): string	ArgumentError - Incorrect or wrong number of arguments
Throw(o: object)	ArgumentError - Incorrect or wrong number of arguments
Time(x): int	ArgumentError - Incorrect or wrong number of arguments
Timeout(ms: int, x): any	ArgumentError - Incorrect or wrong number of arguments Timeout - A time out occurred
ToChar(c: char): char ToChar(i: int): char	ArgumentError - Incorrect or wrong number of arguments
ToInt(c: char): int ToInt(i: int): int ToInt(r: real): int ToInt(s: string): int	ArgumentError - Incorrect or wrong number of arguments
ToList(s: set): list ToList(l: list): list ToList(s: string): list ToList(o: object): list ToList(p: pieceset): list	ArgumentError - Incorrect or wrong number of arguments
ToReal(c: char): real ToReal(i: int): real ToReal(r: real): real ToReal(s: string): real	ArgumentError - Incorrect or wrong number of arguments
ToSet(s: set): set ToSet(l: list): set ToSet(s: string): set ToSet(o: object): set ToSet(p: pieceset): set	ArgumentError - Incorrect or wrong number of arguments
ToString(x): string	ArgumentError - Incorrect or wrong number of arguments
Trap(x): object	ArgumentError - Incorrect or wrong number of arguments
ToString(x): string	ArgumentError - Incorrect or wrong number of arguments

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