/* Prolog Well-formed Pages is an approach to server-side scripting using Prolog which is based on a simple key principle: The source form of a PWP should be WELL-FORMED XML Especially when generating XML rather than HTML, this is such an obvious thing to do. We have many kinds of XML checking tools. - We can tell whether an XML document is WELL FORMED (all the punctuation is right, all tags balance) using practically any decent parser, including SWI Prolog's 'sgml'. - If we can write a Document Type Description then we can check that a document is VALID using tools like Open SP (formerly James Clark's SP) or SWI Prolog's 'sgml'. This does not guarantee that the output will be valid, but it does catch a lot of errors very early. - If we can write an XML Schema then we can check that a document is schema-valid. (SWI Prolog's 'sgml' does not yet come with a schema validator, but who knows what the future holds?). - Since an XML document is just a data structure, we can use any checking tool that we can write in Prolog, IF the input is well-formed so that we can load a template as a Prolog data structure. Having decided that the input should be well formed, that means NO NEW SYNTAX. None of the weird and horrible <% ... %> or whatever not-quite-XML stuff you see in other template systems, making checking so very hard (and therefore, making errors so distressingly common). That in turns means that PWP "markup" must be based on special elements or special attributes. The fact that an XML parser must allow undeclared attributes on any element even when validating, but must not allow undeclared elements, suggests doing this through attributes. In particular, one should be able to take an existing DTD, such as an XHTML DTD, and just use that without modification. So the design reduces to Allow dynamic data selection, insertion, and transformation just using a small number of extra attributes. This description uses the following name space: xmlns:pwp='http://www.cs.otago.ac.nz/staffpriv/ok/pwp.pl' The attributes are pwp:ask = Query pwp:use = Term pwp:how = text | xml pwp:tag = QName or '-' pwp:att = '' | 'one non-alphanumeric character' Here's what they mean. Each element is expanded in the context of a set of variable bindings. After expansion, if the tag is not mapped to '-', all attributes in the pwp: namespace are removed and the children elements are recursively expanded. pwp:ask = Query Query is a Prolog goal. For each solution of Query, the element is further processed with the new variables of Query added to the context. If Query is not a well formed Prolog goal, or if execution of Query throws an exception, page transformation comes to a complete halt and no page is generated. pwp:use = Term pwp:how = text | xml | text-file | xml-file Term is a Prolog term; variables in Term are bound by the context. An empty Term is regarded as a missing value for this attribute. If pwp:how is text, The value of Term is used to define a sequence of characters. - A number produces the same characters that write/1 would. - An atom produces the same characters that write/1 would. - A string produces the same characters that write/1 would. - A list of character codes produces those characters. - The following terms produce the same sequence of characters that the corresponding goal would have sent to the current output stream: + write(Datum) + writeq(Datum) + write_canonical(Datum) + print(Datum) + print(Datum) + format(Format) + format(Format, Arguments) - A singleton list [X] defines the characters that X defines. - Any other term F(T1,...,Tn) defines the characters that T1 defines, followed by the characters that T2 defines, ..., followed by the characters that Tn defines. If pwp:how is xml, The value of Term must be an XML term as defined in the SGML2PL documentation or a list of such terms. A single term is taken as if it had been [Term]. The resulting list of terms replaces the children of the current element and will not be further processed. If pwp:how is text-file, The value of Term is used to define a sequence of characters. That sequence of characters is used as a file name. The file is read as a sequence of characters, and that sequence used as character data. If pwp:how is xml-file, The value of Term is used to define a sequence of characters. That sequence of characters is used as a file name. The file is loaded as XML, and the sequence of XML items thus obtained used. This means that PWP provides XML inclusion without depending on the parser to support XInclude. The default value for pwp:how is text. pwp:tag = QName or '-' If pwp:tag is missing or the value is empty, the current element appears in the output (after further processing) with its present tag. If pwp:tag is a QName, the current element appears (...) with that as its tag. That option is most useful in DTDs, where an "authoring" DTD may use one tag and have it automatically mapped to another tag in the output, e.g., ->
  • . Finally, if pwp:tag is '-', the children of the current element (either the result of pwp:use or the transformed original children, whichever applies) appear in the output but there is no element around them. A missing or empty pwp:ask is just like pwp:ask = 'true'. pwp:att = '' | 'one non-alphanumeric character'. Attributes in the pwp namespace are not affected by this attribute. Such attributes are always stripped out and never substituted into. If pwp:att is missing or empty, attributes of the current element are copied over to the output unchanged. If pwp:att = 'c' for some non-alphanumeric character c, each attribute is examined for occurrences of c(...)c which are as short as possible. There is no one character which could be used every time, so you have to explicitly choose a substitution marker which is safe for the data you do not want to be altered. None of the pwp attributes are inherited, least of all this one. Text outside c(...)c groups is copied unchanged; text inside such a group is parsed as a Prolog term and treated as if by pwp:how = text. Examples: 1. A "Hello World" like example. </head> <body> <p><span pwp:use="Greeting" pwp:tag='-'/></p> </body> </html> where msg.pl contains msg('Hello, World!'). This example illustrates an important point. Prolog Well-Formed Pages provide *NO* way to physically incorporate Prolog *clauses* into a page template. Prolog clauses must be put in separate files which can be checked by a Prolog syntax checker, compiler, cross-referencer, &c WITHOUT the Prolog tool in question needing to know anything whatsoever about PWP. You load the files using pwp:ask on the root element. 2. Binding some variables and using them. <html xmlns:pwp="http://www.cs.otago.ac.nz/staffpriv/ok/pwp.pl"> <head><title>Example 2

    The answer is .

    3. Making a table. We are given a Prolog database staff.pl defining staff(NickName, FullName, Office, Phone, E_Mail_Address). status(NickName, full_time | part_time). We want to make a phone list of full time staff. Phone list for Full-Time staff.

    Phone list for Full-Time staff.

    NamePhone
    4. Substituting into an attribute. Same data base as before, but now we want to make a mailing list page. Phone list for Full-Time staff.

    Phone list for Full-Time staff.

    NameAddress
    5. If-then-else effect. A page that displays the value of the 'SHELL' environment variable if it has one, otherwise displays 'There is no default shell.' $SHELL

    The default shell is .

    There is no default shell.

    There is one other criterion for a good server-side template language: It should be possible to compile templates so as to eliminate most if not all interpretation overhead. This particular notation satisfies that criterion with the limitation that the conversion of a term to character data requires run-time traversal of terms (because the terms are not known until run time). THIS IS A PROOF-OF-CONCEPT IMPLEMENTATION WHICH DOES NOT COMPILE! Is this approach peculiar to Prolog, or could it be used with other languages? Consider Erlang as one possible example. Add attributes ewp:use = Expression ewp:how = text | xml | text-file | xml-file ewp:tag = QName | '-' ewp:att = '' | 'one non-alphanumeric character' just like their pwp: analogues. Also add four more tags: ewp:let = Pattern ewp:be = Expression " exp:in = Expression ewp:st = Expression ... ewp:let = "P" ewp:be = "E" ... evaluates E and binds P to the result. If P doesn't match the value of E, the binding fails and the element is not processed any more. ... ewp:let = "P" ewp:in = "E" ... evaluates E, which should produce a list or tuple. If the result is a tuple, it is converted to a list. For each element of the result list, P is bound to that element, and if it matches, the element is further processed. This provides iteration over a collection. ... ewp:st = "E" ... evaluates E, which should yield 'true' or 'false'. If the result is 'true', processing continues. If the result is 'false', this element is not processed any more. Otherwise an exception occurs. For example, to produce a list of names ending with "Smith": The single Prolog form ewp:ask is broken up into three forms for Erlang, mainly so that the template processor can discover what the variable names are. Would this work only for Erlang? No, it could work for Common Lisp also, using DESTRUCTURING-BIND. It could probably be adpated to Tcl with some effort. Could this kind of approach also work for a statically typed language like Haskell, SML, O'CAML, or F#? Yes, but ONLY via compilation. */ /* The top level is rather crude, because this is really just a proof-of-concept implementation. In particular, error detection and reporting isn't done at all, and this is in no way plugged into a CGI or other web server framework. pwp_files(Input_File_Name, Output_File_Name) reads XML from Input_File_Name, transforms it, and writes the result to Output_File_Name. pwp(Input_Stream, Output_Stream) reads XML from already opened Input_Stream, transforms it, and writes the result to already opened Output_Stream. The core is equally happy with SGML input and HTML output. */ :- use_module(library(sgml), [load_xml_file/2]). :- use_module(library(sgml_write), [xml_write/3]). % pwp_files(+In: atom, +Out: atom) % loads an Xml document from the file named In, % transforms it using the PWP attributes, and % writes the transformed version to the new file named Out. pwp_files(In, Out) :- open(In, read, Input), open(Out, write, Output), pwp(Input, Output), close(Output), close(Input). % pwp(+Input: input_stream, +Output: output_stream) % loads an Xml document from the given Input stream, % transforms it using the PWP attributes, and % writes the transformed version to the given Output stream. pwp(Input, Output) :- load_xml_file(stream(Input), Contents), pwp_list(Contents, Transformed, []), !, xml_write(Output, Transformed, []). /* Recall that an XML term is one of Character Data number(C) A character which won't fit in the supported character set. Obsolete in current SWI, which supports Unicode. sdata(...) SDATA (SGML only) ndata(...) NDATA pi(...) Processing instruction element(Name, [Att...], [Child...]) where Att is Attribute=Value and Child is an XML term. We are only concerned with elements; all other XML terms are left alone. I have given some thought to recognising processing instructions, executing the Command, and removing the processing instructions, as a debugging tool. But this is a proof-of-concept implementation; debugging features can wait for The Real Thing. */ % pwp_list(+list(xml), -list(xml), +Context) % maps down a list of XML items, acting specially on elements and % copying everything else unchanged, including white space. % The Context is a list of 'VariableName'=CurrentValue bindings. pwp_list([], [], _). pwp_list([element(Tag0,Atts0,Kids0)|Xs], Ys0, Context) :- !, pwp_attributes(Atts0, Ask, Use, How, Att, Tag1, Atts1), ( nonvar(Tag1), Tag1 \== '' -> Tag2 = Tag1 ; Tag2 = Tag0 ), ( nonvar(Ask), Ask \== '', Ask \== 'true' -> atom_to_term(Ask, Query, Bindings), pwp_unite(Bindings, Context, Context1), findall(Xml, (Query, pwp_element(Tag2, Atts1, Kids0, Use, How, Att, Context1, Xml)), New_Content) ;/* Ask is missing, empty, or true */ pwp_element(Tag2, Atts1, Kids0, Use, How, Att, Context, New_Content) ), pwp_attach(New_Content, Ys0, Ys1), pwp_list(Xs, Ys1, Context). pwp_list([X|Xs], [X|Ys], Context) :- pwp_list(Xs, Ys, Context). % pwp_attributes(+Atts0: list(=(atom,atom)), % -Ask: atom, -Use: atom, -How: atom, -Att: atom, -Tag: atom, % -Atts1: list(=(atom,atom)) % walks down a list of AttributeName=ItsValue pairs, stripping out those % whose AttributeName begins with the 'pwp:' prefix, and copying the % rest to Atts1. Along the way, Ask/Use/How/Att/Tag are bound to the % values of the pwp:ask/pwp:use/pwp:how/pwp:att/pwp:tag attributes, % if any. At the end, any of these variables that are still unbound % REMAIN unbound; they are not bound to default values. pwp_attributes([], _, _, _, _, _, []). pwp_attributes([AV|AVs], Ask, Use, How, Att, Tag, New_Atts1) :- AV = (Name=Value), ( Name = 'xmlns:pwp' -> New_Atts1 = New_Atts2 ; Name = 'pwp:ask' -> Ask = Value, New_Atts1 = New_Atts2 ; Name = 'pwp:use' -> Use = Value, New_Atts1 = New_Atts2 ; Name = 'pwp:how' -> How = Value, New_Atts1 = New_Atts2 ; Name = 'pwp:att' -> Att = Value, New_Atts1 = New_Atts2 ; Name = 'pwp:tag' -> Tag = Value, New_Atts1 = New_Atts2 ; atom_codes(Name, [0'p,0'w,0'p,0':|_]) -> New_Atts1 = New_Atts2 ; New_Atts1 = [AV|New_Atts2] ), pwp_attributes(AVs, Ask, Use, How, Att, Tag, New_Atts2). % pwp_unite(+Bindings, +Context0, -Context: list(=(atom,any))) % merges the new Bindings with the bindings in the outer Context0, % constructing a new list of VariableName=CurrentValue bindings in % Context1. This is only used when the CurrentValue parts of the % new Bindings are known to be distinct new variables, so the % Bindings cannot possibly conflict with any existing binding in % Context0. This is O(|Bindings|.|Context0|), which is not that % efficient, but since we do not expect there to be very many % variables it doesn't matter much. pwp_unite(Bindings, Context0, Context) :- pwp_unite(Bindings, Context0, Context0, Context). pwp_unite([], _, Context, Context). pwp_unite([Binding|Bindings], Context0, Context1, Context) :- memberchk(Binding, Context0), !, pwp_unite(Bindings, Context0, Context1, Context). pwp_unite([Binding|Bindings], Context0, Context1, Context) :- pwp_unite(Bindings, Context0, [Binding|Context1], Context). % pwp_unite(+Bindings, +Context0: list(=(atom,any))) % looks up the bindings in Bindings in the outer Context0. % This is only used for 'pwp:use' terms (and the related terms % in $(...)$ attribute value substitutions), so that we have % no interest in forming a new context. (If we did, we'd use % pwp_unite/3 instead.) This is only used when the CurrentValue % parts of the new Bindings are known to be distinct new variables, % so the Bindings cannot possibly conflict with any existing % binding in Context0. However, there _could_ be new variables % in Bindings, and that would cause problems. An XML term may % not contain variables, and a term we want to convert to a list % of character codes had better not contain variables either. % One approach would be to just bind such variables to something, % another is to throw some kind of exception. For the moment we % call functor/3 so as to get an instantiation error. pwp_unite([], _). pwp_unite([Binding|Bindings], Context) :- memberchk(Binding, Context), !, pwp_unite(Bindings, Context). pwp_unite([_=Value|_], _) :- functor(Value, _, _). % pwp_attach(+Tree, ?Ys0: list(xml), ?Ys: list(xml)) % is a combination of "flatten" and "append". % It unifies Ys0\Ys with the result of flattening Tree. pwp_attach([], Ys, Ys) :- !. pwp_attach([X|Xs], Ys0, Ys) :- !, pwp_attach(X, Ys0, Ys1), pwp_attach(Xs, Ys1, Ys). pwp_attach(X, [X|Ys], Ys). pwp_element('-', _, Kids, Use, How, _, Context, Xml) :- !, pwp_use(Use, How, Kids, Context, Xml). pwp_element(Tag, Atts, Kids, Use, How, Magic, Context, element(Tag,Atts1,Kids1)) :- ( nonvar(Magic), atom_codes(Magic, [C]) -> pwp_substitute(Atts, C, Context, Atts1) ; Atts1 = Atts ), pwp_use(Use, How, Kids, Context, Kids1). pwp_use('', _, Kids, Context, Kids1) :- !, pwp_list(Kids, Kids1, Context). pwp_use(Use, How, _, Context, Kids1) :- atom_to_term(Use, Term, Bindings), pwp_unite(Bindings, Context), pwp_how(How, Term, Kids1). pwp_how('text', Term, [CData]) :- !, pwp_use_codes(Term, Codes, []), atom_codes(CData, Codes). pwp_how('xml', Term, Kids1) :- ( Term == [] -> Kids1 = Term ; Term = [_|_] -> Kids1 = Term ; Kids1 = [Term] ). pwp_how('text-file', Term, [CData]) :- pwp_use_codes(Term, Codes, []), atom_codes(FileName, Codes), read_file_to_codes(FileName, FileCodes, []), atom_codes(CData, FileCodes). pwp_how('xml-file', Term, Kids1) :- pwp_use_codes(Term, Codes, []), atom_codes(FileName, Codes), load_xml_file(FileName, Kids1). pwp_substitute([], _, _, []). pwp_substitute([AV|AVs], C, Context, [AV1|Atts1]) :- AV = (Name = Value), atom_codes(Value, Codes), pwp_split(Codes, C, B0, T0, A0), !, pwp_substitute(B0, T0, A0, C, Context, V), atom_codes(New_Value, V), AV1 = (Name = New_Value), pwp_substitute(AVs, C, Context, Atts1). pwp_substitute([AV|AVs], C, Context, [AV|Atts1]) :- pwp_substitute(AVs, C, Context, Atts1). pwp_substitute(B0, T0, A0, C, Context, V0) :- append(B0, V1, V0), atom_codes(Atom, T0), atom_to_term(Atom, Term, Bindings), pwp_unite(Bindings, Context, _), pwp_use_codes(Term, V1, V2), ( pwp_split(A0, C, B1, T1, A1) -> pwp_substitute(B1, T1, A1, C, Context, V2) ; V2 = A0 ). pwp_split(Codes, C, Before, Text, After) :- append(Before, [C,0'(|Rest], Codes), append(Text, [0'),C|After], Rest), !. pwp_use_codes(format(Format), S0, S) :- !, pwp_format(Format, [], S0, S). pwp_use_codes(format(Format,Args), S0, S) :- !, pwp_format(Format, Args, S0, S). pwp_use_codes(write_canonical(Datum), S0, S) :- !, pwp_format('~k', [Datum], S0, S). pwp_use_codes(print(Datum), S0, S) :- !, pwp_format('~p', [Datum], S0, S). pwp_use_codes(writeq(Datum), S0, S) :- !, pwp_format('~q', [Datum], S0, S). pwp_use_codes(write(Datum), S0, S) :- !, pwp_format('~w', [Datum], S0, S). pwp_use_codes(Atomic, S0, S) :- atomic(Atomic), !, ( number(Atomic) -> number_codes(Atomic, Codes) ; atom(Atomic) -> atom_codes(Atomic, Codes) ; string(Atomic) -> string_to_list(Atomic, Codes) ; pwp_format('~w', [Atomic], S0, S) ), append(Codes, S, S0). pwp_use_codes([X|Xs], S0, S) :- pwp_is_codes([X|Xs]), !, append([X|Xs], S, S0). pwp_use_codes([X|Xs], S0, S) :- !, pwp_use_codes(Xs, X, S0, S). pwp_use_codes(Compound, S0, S) :- Compound =.. [_,X|Xs], pwp_use_codes(Xs, X, S0, S). pwp_use_codes([], X, S0, S) :- !, pwp_use_codes(X, S0, S). pwp_use_codes([Y|Ys], X, S0, S) :- pwp_use_codes(X, S0, S1), pwp_use_codes(Ys, Y, S1, S). % pwp_is_codes(+String: any) % is true when String is a list of integers and each of those % integers is a possible Unicode value (in the range U+0000..U+10FFFF). % Back in the days of ISO Latin 1 we would have checked for 0..255, % and way back in the days of ASCII for 0..127. Yes, there are more % than a million possible characters in Unicode; currently about % 100 000 of them are in use. pwp_is_codes([]). pwp_is_codes([C|Cs]) :- integer(C), C >= 0, C =< 1114111, pwp_is_codes(Cs). % What we really REALLY want is for format/[3] to recognise % format(chars(S), Fmt, Args) :- format(chars(S,[]), Fmt, Args). % format(chars(S0,S), Fmt, Args) % format(codes(S), Fmt, Args) :- format(codes(S,[]), Fmt, Args). % format(codes(S0,S), Fmt, Args) % But there is no such animal. SWI Prolog *does*, however, have % sformat/3, which returns a string, which leaves us wishing for % string_to_codes(String, Codes0, Codes) and % string_to_chars(String, Chars0, Chars), % but those don't exist either. So we have to make a string we % don't want, and then make a list we don't want, just in order % to get the difference list we DO want. Without sformat/3, we % would have to do something desperate like writing to a % temporary file... pwp_format(Format, Arguments, S0, S) :- sformat(String, Format, Arguments), string_to_list(String, Codes), append(Codes, S, S0). pwp_test :- pwp_files('pwp1.xml', 'pwp1.htm'), pwp_files('pwp2.xml', 'pwp2.htm'), pwp_files('pwp3.xml', 'pwp3.htm'), pwp_files('pwp4.xml', 'pwp4.htm'), pwp_files('pwp5.xml', 'pwp5.htm'). /* +(1) :- pwp_files('pwp1.xml', '/dev/tty'). +(2) :- pwp_files('pwp2.xml', '/dev/tty'). +(3) :- pwp_files('pwp3.xml', '/dev/tty'). +(4) :- pwp_files('pwp4.xml', '/dev/tty'). +(5) :- pwp_files('pwp5.xml', '/dev/tty'). +(6) :- pwp_files('pwp6.xml', '/dev/tty'). +(7) :- pwp_files('pwp7.xml', '/dev/tty'). +(8) :- pwp_files('pwp8.xml', '/dev/tty'). */