Note: The following enhancements to or deviations from Tutorial D<\/b> should be considered experimental. Depending on their value (or lack thereof) they may or may not be included in future versions of Rel<\/i>.<\/p>\n
Catalog:<\/b>Rel<\/i> provides a simple system catalog, via a predefined relvar called “sys.Catalog”. The contents of the Catalog, i.e., the relvars currently defined in a database, may be displayed by retrieving the contents of the sys.Catalog relvar. For example:<\/p>\n A relation of all defined relvar names can be obtained using:<\/p>\n Case sensitivity:<\/b>Rel<\/i> language keywords are not case sensitive. By convention, keywords are shown here in upper case to visually distinguish them, but the parser recognises them in lower case as well. Rel<\/i> identifiers and string comparisons are case sensitive.<\/p>\n OUTPUT, WRITE, and WRITELN statements:<\/b> Statements have been introduced to send raw text to the client. For example, the following:<\/p>\n Will send the following to the client:<\/p>\n OUTPUT, WRITE, and WRITELN each have slightly different behaviour. OUTPUT sends text in a form that can be parsed by Rel<\/i>, followed by a new line. WRITELN sends text in a natural human-readable form, followed by a new line. WRITE is the same as WRITELN, but does not send a new line. In the current implementation, the only significant difference between OUTPUT and WRITELN is that OUTPUT will present a CHAR surrounded by double quotes, whereas WRITELN will not include the double quotes.<\/p>\n WRITELN and OUTPUT may be specified with no expression, in order to send a new line. Eg.:<\/p>\n DROP OPERATOR:<\/b> Since operators may be overloaded, an alteration to the specified grammar is required to disambiguate the desired operator. For example, the following operator definitions are allowable:<\/p>\n Dropping the first operator is performed with the following:<\/p>\n Dropping the second is performed with the following:<\/p>\n Compile-time announcements:<\/b> A statement has been provided to output strings during compilation. This may be used to provide compilation progress indicators in lengthy scripts. Eg.:<\/p>\n Comments:<\/b> Comments are specified using conventional C++ and Java syntax. Eg:<\/p>\n ORDER:<\/b> An ORDER operator has been implemented to accept a relation and return a row-ordered ARRAY. This is useful for generating human-readable output. For example:<\/p>\n ARRAY element dereference:<\/b> As defined in the Tutorial D<\/b> grammar, ARRAY elements are dereferenced with “(…)”, e.g.:<\/p>\n However, Rel<\/i> defines ARRAY element dereference as “[…]”. The above will not work. Instead, use:<\/p>\n This avoids ambiguity with operator invocation, and follows a familiar convention that users of C, C++, C#, Java, and a number of other popular general-purpose programming languages are likely to recognise.<\/p>\n FOR:<\/b> The ‘FOR’ statement permits iterating over the tuples in an ARRAY and executing arbitrary code. For example:<\/p>\n The above example is shorthand for:<\/p>\n FOR is typically used conjunction with ORDER (see above<\/i>) to permit iterating tuples from a relational expression in a specified sequence, but if no particular sequence is required, ORDER may be specified as shown in the following example:<\/p>\n EXECUTE:<\/b> The EXECUTE statement permits run-time execution of dynamically-generated Tutorial D<\/b> code. For example, the following uses EXECUTE to emit Tutorial D<\/b>definitions of user-defined real relvars, by dynamically generating and executing an ‘OUTPUT’ statement for each relvar using the relvar name obtained at run-time:<\/p>\n Note that EXECUTE runs in the global scope, rather than the local scope in which it is used.<\/p>\n User-defined OPERATORs written in Java:<\/b> OPERATORs may be defined in Java. See the Scripts directory of a Rel<\/i> installation for examples.<\/p>\n User-defined TYPEs written in Java:<\/b> New built-in TYPEs may be defined using Java within Rel<\/i>. See the Scripts directory of a Rel<\/i> installation for examples.<\/p>\n","protected":false},"excerpt":{"rendered":" Language Enhancements Note: The following enhancements to or deviations from Tutorial D should be considered experimental. Depending on their value (or lack thereof) they may or may not be included in future versions of Rel. Catalog:Rel provides a simple system catalog, via a predefined relvar called “sys.Catalog”. The contents of the Catalog, i.e., the relvars Read more about Language Enhancements<\/span>[…]<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"parent":17,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":"","_links_to":"","_links_to_target":""},"class_list":["post-48","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/reldb.org\/c\/index.php\/wp-json\/wp\/v2\/pages\/48","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/reldb.org\/c\/index.php\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/reldb.org\/c\/index.php\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/reldb.org\/c\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/reldb.org\/c\/index.php\/wp-json\/wp\/v2\/comments?post=48"}],"version-history":[{"count":1,"href":"https:\/\/reldb.org\/c\/index.php\/wp-json\/wp\/v2\/pages\/48\/revisions"}],"predecessor-version":[{"id":49,"href":"https:\/\/reldb.org\/c\/index.php\/wp-json\/wp\/v2\/pages\/48\/revisions\/49"}],"up":[{"embeddable":true,"href":"https:\/\/reldb.org\/c\/index.php\/wp-json\/wp\/v2\/pages\/17"}],"wp:attachment":[{"href":"https:\/\/reldb.org\/c\/index.php\/wp-json\/wp\/v2\/media?parent=48"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}\n
sys.Catalog\r\n<\/code><\/pre>\n<\/blockquote>\n\n
sys.Catalog {Name}\r\n<\/code><\/pre>\n<\/blockquote>\n\n
VAR i INTEGER;\r\nDO i := 1 TO 3;\r\n\tOUTPUT i;\r\nEND;\r\n<\/code><\/pre>\n<\/blockquote>\n\n
1\r\n2\r\n3\r\n<\/code><\/pre>\n<\/blockquote>\n\n
WRITELN;\r\n<\/code><\/pre>\n<\/blockquote>\n\n
OPERATOR foo(x INTEGER, y INTEGER) RETURNS INTEGER;\r\n\tRETURN x + y;\r\nEND OPERATOR;\r\n\r\nOPERATOR foo(x INTEGER) RETURNS INTEGER;\r\n\tRETURN x + 2;\r\nEND OPERATOR;\r\n<\/code><\/pre>\n<\/blockquote>\n\n
DROP OPERATOR foo(INTEGER, INTEGER);\r\n<\/code><\/pre>\n<\/blockquote>\n\n
DROP OPERATOR foo(INTEGER);\r\n<\/code><\/pre>\n<\/blockquote>\n\n
ANNOUNCE 'Operator fibonacci (INTEGER) INTEGER being created...';\r\nOPERATOR fibonacci (r INTEGER) RETURNS INTEGER;\r\n IF r <= 1 THEN\r\n RETURN r;\r\n ELSE\r\n RETURN fibonacci(r - 1) + fibonacci(r - 2);\r\n END IF;\r\nEND OPERATOR;\r\n<\/code><\/pre>\n<\/blockquote>\n\n
\/\/ This is a comment line\r\n\r\n\/* This is a multi-line\r\n comment block *\/\r\n<\/code><\/pre>\n<\/blockquote>\n\n
WRITELN P ORDER (DESC WEIGHT, ASC P#);\r\n<\/code><\/pre>\n<\/blockquote>\n\n
VAR catalog ARRAY SAME_TYPE_AS(TUPLE FROM sys.Catalog {Name});\r\nLOAD catalog FROM sys.Catalog {Name} ORDER (ASC Name);\r\nOUTPUT catalog(3);\r\n<\/code><\/pre>\n<\/blockquote>\n\n
VAR catalog ARRAY SAME_TYPE_AS(TUPLE FROM sys.Catalog {Name});\r\nLOAD catalog FROM sys.Catalog {Name} ORDER (ASC Name);\r\nOUTPUT catalog[3];\r\n<\/code><\/pre>\n<\/blockquote>\n\n
FOR sys.Catalog {Name} ORDER(ASC Name); \r\n WRITELN \"Name = \" || Name;\r\nEND FOR;\r\n<\/code><\/pre>\n<\/blockquote>\n\n
VAR catalog ARRAY SAME_TYPE_AS(TUPLE FROM sys.Catalog {Name});\r\nLOAD catalog FROM sys.Catalog {Name} ORDER (ASC Name);\r\nVAR i INTEGER;\r\nDO i := 0 TO COUNT(catalog) - 1;\r\n BEGIN;\r\n VAR Name INIT(Name FROM catalog[i]);\r\n WRITELN \"Name = \" || Name;\r\n END;\r\nEND DO;\r\n<\/code><\/pre>\n<\/blockquote>\n\n
FOR sys.Catalog {Name} ORDER(); \r\n WRITELN \"Name = \" || Name;\r\nEND FOR;\r\n<\/code><\/pre>\n<\/blockquote>\n\n
FOR sys.Catalog WHERE Owner <> \"Rel\" AND NOT isVirtual ORDER();\r\n BEGIN;\r\n\tWRITELN \"VAR \" || Name || \" REAL \" || Definition || \";\";\r\n\tWRITE Name || \" := \";\r\n\tEXECUTE \"OUTPUT \" || Name || \";\";\r\n\tWRITELN \";\";\r\n END;\r\nEND FOR;\r\n<\/code><\/pre>\n<\/blockquote>\n