I am trying to wrap my head around an assignment question, therefore I would very highly appreciate any help in the right direction (and not necessarily a complete answer). I am being asked to write the grammar specification for this parser. The specification for the grammar that I must implement can be found here:
http://anoopsarkar.github.io/compilers-class/decafspec.html
Although the documentation is there, I do not understand a few things, such as how to write (in my .y file) things such as
{ identifier },+
I understand that this would mean a comma-separated list of 1 (or more) occurrences of an identifier, however when I write it as such, the compiler displays an error of unrecognized symbols '+' and ',', being mistaken as whitespace. I tried '{' identifier "},+", but I haven't the slightest clue whether that is correct or not.
I have written the lexical analyzer portion (as it was from the previous segment of the assignment) which returns tokens (T_ID, T_PLUS, etc.) accordingly, however there is this new notion that I must assign 'yylval' to be the value of the token itself. To my understanding, this is only necessary if I am in need of the actual value of the token, therefore I would need the value of an identifier token T_ID, but not necessarily the value of T_PLUS, being '+'. This is done by creating a %union in the parser generator file, which I have done, and have provided the tokens that I currently believe would require the literal token value with the proper yylval assignment.
Here is my lexical analysis code (I could not get it to format properly, I apologize): https://pastebin.com/XMZwvWCK
Here is my parser file decafast.y: https://pastebin.com/2jvaBFQh
And here is the final piece of code supplied to me, the C++ code to build an abstract syntax tree at the end:
https://pastebin.com/ELy53VrW?fbclid=IwAR2cFT_-pGKlVZ2liC-zAe3Fw0BWDlGjrrayqEGV4JuJq1_7nKoe9-TLTlA
To finalize my question, I do not know if I am creating my grammar rules correctly. I have tried my best to follow the specification in the above website, but I can't help but feel that what I am writing is completely wrong. My compiler is spitting out nothing but "warning: rule useless in grammar" for almost every (if not every) rule.
If anyone could help me out and point me in the right direction on how to make any progress, I would highly, highly appreciate it.
The decaf specification is written in (an) Extended Backus Naur Form (EBNF), which includes a number of convenience operators for repetition, optionality and grouping. These are not part of the bison/yacc syntax, which is pretty well limited to BNF. (Bison/yacc do allow the alternation operator |, but since there is no way to group subpatterns, alteration can only be used at the top-level, to combine two productions for the same non-terminal.)
The short section at the beginning of the specification which describes EBNF includes a grammar for the particular variety of EBNF that is being used. (Since this grammar is itself recursively written in the same EBNF, there is a need to apply a bit of inductive reasoning.) When it says, for example,
CommaList = "{" Expression "}+," .
it is not saying that "}+," is the peculiar spelling of a comma-repetition operator. What it is saying is that when you see something in the Decaf grammar surrounded by { and }+,, that should be interpreted as describing a comma-separated list.
For example, the Decaf grammar includes:
FieldDecl = var { identifier }+, Type ";" .
That means that a FieldDecl can be (amongst other possibilities) the token var followed by a comma-separated list of identifier tokens and then followed by a Type and finally a semicolon.
As I said, bison/yacc don't implement the EBNF operators, so you have to find an equivalent yourself. Since BNF doesn't allow any form of grouping -- and a list is a grouped subexpression -- we need to rewrite the subexpression of a production as a new non-terminal. Also, I suppose we need to use the tokens defined in spec (although bison allows a more readable syntax).
So to yacc-ify this EBNF production, we first introducing the new non-terminal and replace the token names:
FieldDecl: T_VAR IdentifierList Type T_SEMICOLON
Which leaves the definition of IdentifierList. Repetition in BNF is always produced with recursion, following a very simple model which uses two productions:
the base, which is the simplest possible repetition (usually either nothing or a single list item), and
the recursion, which describes a longer possibility by extending a shorter one.
In this case, the list must have at least one item, and we extend by adding a comma and another item:
IdentifierList
: T_ID /* base case */
| IdentifierList T_COMMA T_ID /* Recursive extension */
The point of this exercise is to develop your skills in thinking grammatically: that is, factoring out the syntax and semantics of the language. So you should try to understand the grammars presented, both for Decaf and for the author's version of EBNF, and avoid blindly copying code (including grammars). Good luck!
I am trying to write an antlr4 grammar for a customized language which among its lexer rules originally contained the following:
PLUS : '+' ;
MINUS : '-' ;
NUMBER: ('+'|'-')? [0-9]+ ;
COMMENT : '/*' (COMMENT|~'*'|('*' ~'/'))* '*/' -> skip ;
WS : (' ' | '\t' | '\n') -> skip ;
The parser grammar contains, among other things, an arithmetic expression evaluator, and what I found is that using these lexer rules failed to parse the input '2-2' correctly, which should come out as NUMBER MINUS NUMBER, and instead just returned two NUMBER tokens. I therefore broke out the unary + and - applications into it's own parser rule, as follows:
literal_number : NUMBER
| '-' NUMBER
| '+' NUMBER ;
And defined NUMBER simply as:
NUMBER: [0-9]+ ;
However, with this arrangement, the literal_number parser rule is being activated even if there is whitespace or comments between the plus and minus tokens and the number itself. This should not be valid in parser contexts where I am expecting to see only an integer constant (which is actually anywhere other than when parsing arithmetic expressions). I have another parser rule elsewhere in my parser grammar that handles unary negation already, so I do not need to replicate that in the literal_number parser rule anyways, so all I what I want is for the literal_number parser rule to refer only to places in the text where a real integer constant had been found.
How can I do this? I have already looked at questions on stackoverflow pertaining to rules that are sensitive to whitespace, but I have not been able to figure out how to apply any of those solutions to my problem.
I'm not sure that this matters for my question, but my target language is c++, although I expect I may still be able to generalize from a java-specific example if one is offered.
EDIT:
The response that I've seen so far highlights an issue with my original comment which may have been ambiguous. In my defense I had not wanted to complicate my original question with information that I did not immediately see as relevant, but in light of the response I've seen so far, I can now clearly see that it is. I can only offer my apologies for this initial oversight.
In addition to the literal_number rule, I also have the following rule for expressions, which, in particular, has a rule allowing for negation.
expression : ID # look up value
| literal_number # number
| MINUS expression # negate
| expression (STAR|SLASH) expression # multiply
| expression (PLUS|MINUS) expression # add
;
So to that end, the expression 2-2 should evaluate as literal_number (2) MINUS literal_number (2), 2--2 should evaluate as literal_number (2) MINUS literal_number (-2), while 2-- 2 should evaluate as literal_number (2) MINUS MINUS literal_number (2).
So basically, as I said originally, I only want the literal_number rule to be used at all when the NUMBER is by itself or MINUS and the NUMBER are side by side with no ignored tokens between them, but I cannot just make ('+'|'-') [0-9] a lexical rule for NUMBER without causing the problem I had in the first place.
I have this trouble: I must verify the correctness of many mathematical expressions especially check for consecutive operators + - * /.
For example:
6+(69-9)+3
is ok while
6++8-(52--*3)
no.
I am not using the library <regex> since it is only compatible with C++11.
Is there a alternative method to solve this problem? Thanks.
You can use a regular expression to verify everything about a mathematical expression except the check that parentheses are balanced. That is, the regular expression will only ensure that open and close parentheses appear at the point in the expression they should appear, but not their correct relationship with other parentheses.
So you could check both that the expression matches a regex and that the parentheses are balanced. Checking for balanced parentheses is really simple if there is only one type of parenthesis:
bool check_balanced(const char* expr, char open, char close) {
int parens = 0;
for (const char* p = expr; *p; ++p) {
if (*p == open) ++parens;
else if (*p == close && parens-- == 0) return false;
}
return parens == 0;
}
To get the regular expression, note that mathematical expressions without function calls can be summarized as:
BEFORE* VALUE AFTER* (BETWEEN BEFORE* VALUE AFTER*)*
where:
BEFORE is sub-regex which matches an open parenthesis or a prefix unary operator (if you have prefix unary operators; the question is not clear).
AFTER is a sub-regex which matches a close parenthesis or, in the case that you have them, a postfix unary operator.
BETWEEN is a sub-regex which matches a binary operator.
VALUE is a sub-regex which matches a value.
For example, for ordinary four-operator arithmetic on integers you would have:
BEFORE: [-+(]
AFTER: [)]
BETWEEN: [-+*/]
VALUE: [[:digit:]]+
and putting all that together you might end up with the regex:
^[-+(]*[[:digit:]]+[)]*([-+*/][-+(]*[[:digit:]]+[)]*)*$
If you have a Posix C library, you will have the <regex.h> header, which gives you regcomp and regexec. There's sample code at the bottom of the referenced page in the Posix standard, so I won't bother repeating it here. Make sure you supply REG_EXTENDED in the last argument to regcomp; REG_EXTENDED|REG_NOSUB, as in the example code, is probably even better since you don't need captures and not asking for them will speed things up.
You can loop over each charin your expression.
If you encounter a + you can check whether it is follow by another +, /, *...
Additionally you can group operators together to prevent code duplication.
int i = 0
while(!EOF) {
switch(expression[i]) {
case '+':
case '*': //Do your syntax checks here
}
i++;
}
Well, in general case, you can't solve this with regex. Arithmethic expressions "language" can't be described with regular grammar. It's context-free grammar. So if what you want is to check correctness of an arbitrary mathemathical expression then you'll have to write a parser.
However, if you only need to make sure that your string doesn't have consecutive +-*/ operators then regex is enough. You can write something like this [-+*/]{2,}. It will match substrings with 2 or more consecutive symbols from +-*/ set.
Or something like this ([-+*/]\s*){2,} if you also want to handle situations with spaces like 5+ - * 123
Well, you will have to define some rules if possible. It's not possible to completely parse mathamatical language with Regex, but given some lenience it may work.
The problem is that often the way we write math can be interpreted as an error, but it's really not. For instance:
5--3 can be 5-(-3)
So in this case, you have two choices:
Ensure that the input is parenthesized well enough that no two operators meet
If you find something like --, treat it as a special case and investigate it further
If the formulas are in fact in your favor (have well defined parenthesis), then you can just check for repeats. For instance:
--
+-
+*
-+
etc.
If you have a match, it means you have a poorly formatted equation and you can throw it out (or whatever you want to do).
You can check for this, using the following regex. You can add more constraints to the [..][..]. I'm giving you the basics here:
[+\-\*\\/][+\-\*\\/]
which will work for the following examples (and more):
6++8-(52--*3)
6+\8-(52--*3)
6+/8-(52--*3)
An alternative, probably a better one, is just write a parser. it will step by step process the equation to check it's validity. A parser will, if well written, 100% accurate. A Regex approach leaves you to a lot of constraints.
There is no real way to do this with a regex because mathematical expressions inherently aren't regular. Heck, even balancing parens isn't regular. Typically this will be done with a parser.
A basic approach to writing a recursive-descent parser (IMO the most basic parser to write) is:
Write a grammar for a mathematical expression. (These can be found online)
Tokenize the input into lexemes. (This will be done with a regex, typically).
Match the expressions based on the next lexeme you see.
Recurse based on your grammar
A quick Google search can provide many example recursive-descent parsers written in C++.
I've posted this on the D newsgroup some months ago, but for some reason, the answer never really convinced me, so I thought I'd ask it here.
The grammar of D is apparently context-free.
The grammar of C++, however, isn't (even without macros). (Please read this carefully!)
Now granted, I know nothing (officially) about compilers, lexers, and parsers. All I know is from what I've learned on the web.
And here is what (I believe) I have understood regarding context, in not-so-technical lingo:
The grammar of a language is context-free if and only if you can always understand the meaning (though not necessarily the exact behavior) of a given piece of its code without needing to "look" anywhere else.
Or, in even less rigor:
The grammar cannot be context-free if I need I can't tell the type of an expression just by looking at it.
So, for example, C++ fails the context-free test because the meaning of confusing<sizeof(x)>::q < 3 > (2) depends on the value of q.
So far, so good.
Now my question is: Can the same thing be said of D?
In D, hashtables can be created through a Value[Key] declaration, for example
int[string] peoplesAges; // Maps names to ages
Static arrays can be defined in a similar syntax:
int[3] ages; // Array of 3 elements
And templates can be used to make them confusing:
template Test1(T...)
{
alias int[T[0]] Test;
}
template Test2(U...)
{
alias int[U] Test2; // LGTM
}
Test1!(5) foo;
Test1!(int) bar;
Test2!(int) baz; // Guess what? It's invalid code.
This means that I cannot tell the meaning of T[0] or U just by looking at it (i.e. it could be a number, it could be a data type, or it could be a tuple of God-knows-what). I can't even tell if the expression is grammatically valid (since int[U] certainly isn't -- you can't have a hashtable with tuples as keys or values).
Any parsing tree that I attempt to make for Test would fail to make any sense (since it would need to know whether the node contains a data type versus a literal or an identifier) unless it delays the result until the value of T is known (making it context-dependent).
Given this, is D actually context-free, or am I misunderstanding the concept?
Why/why not?
Update:
I just thought I'd comment: It's really interesting to see the answers, since:
Some answers claim that C++ and D can't be context-free
Some answers claim that C++ and D are both context-free
Some answers support the claim that C++ is context-sensitive while D isn't
No one has yet claimed that C++ is context-free while D is context-sensitive :-)
I can't tell if I'm learning or getting more confused, but either way, I'm kind of glad I asked this... thanks for taking the time to answer, everyone!
Being context free is first a property of generative grammars. It means that what a non-terminal can generate will not depend on the context in which the non-terminal appears (in non context-free generative grammar, the very notion of "string generated by a given non-terminal" is in general difficult to define). This doesn't prevent the same string of symbols to be generated by two non-terminals (so for the same strings of symbols to appear in two different contexts with a different meaning) and has nothing to do with type checking.
It is common to extend the context-free definition from grammars to language by stating that a language is context-free if there is at least one context free grammar describing it.
In practice, no programming language is context-free because things like "a variable must be declared before it is used" can't be checked by a context-free grammar (they can be checked by some other kinds of grammars). This isn't bad, in practice the rules to be checked are divided in two: those you want to check with the grammar and those you check in a semantic pass (and this division also allows for better error reporting and recovery, so you sometimes want to accept more in the grammar than what would be possible in order to give your users better diagnostics).
What people mean by stating that C++ isn't context-free is that doing this division isn't possible in a convenient way (with convenient including as criteria "follows nearly the official language description" and "my parser generator tool support that kind of division"; allowing the grammar to be ambiguous and the ambiguity to be resolved by the semantic check is an relatively easy way to do the cut for C++ and follow quite will the C++ standard, but it is inconvenient when you are relying on tools which don't allow ambiguous grammars, when you have such tools, it is convenient).
I don't know enough about D to know if there is or not a convenient cut of the language rules in a context-free grammar with semantic checks, but what you show is far from proving the case there isn't.
The property of being context free is a very formal concept; you can find a definition here. Note that it applies to grammars: a language is said to be context free if there is at least one context free grammar that recognizes it. Note that there may be other grammars, possibly non context free, that recognize the same language.
Basically what it means is that the definition of a language element cannot change according to which elements surround it. By language elements I mean concepts like expressions and identifiers and not specific instances of these concepts inside programs, like a + b or count.
Let's try and build a concrete example. Consider this simple COBOL statement:
01 my-field PICTURE 9.9 VALUE 9.9.
Here I'm defining a field, i.e. a variable, which is dimensioned to hold one integral digit, the decimal point, and one decimal digit, with initial value 9.9 . A very incomplete grammar for this could be:
field-declaration ::= level-number identifier 'PICTURE' expression 'VALUE' expression '.'
expression ::= digit+ ( '.' digit+ )
Unfortunately the valid expressions that can follow PICTURE are not the same valid expressions that can follow VALUE. I could rewrite the second production in my grammar as follows:
'PICTURE' expression ::= digit+ ( '.' digit+ ) | 'A'+ | 'X'+
'VALUE' expression ::= digit+ ( '.' digit+ )
This would make my grammar context-sensitive, because expression would be a different thing according to whether it was found after 'PICTURE' or after 'VALUE'. However, as it has been pointed out, this doesn't say anything about the underlying language. A better alternative would be:
field-declaration ::= level-number identifier 'PICTURE' format 'VALUE' expression '.'
format ::= digit+ ( '.' digit+ ) | 'A'+ | 'X'+
expression ::= digit+ ( '.' digit+ )
which is context-free.
As you can see this is very different from your understanding. Consider:
a = b + c;
There is very little you can say about this statement without looking up the declarations of a,b and c, in any of the languages for which this is a valid statement, however this by itself doesn't imply that any of those languages is not context free. Probably what is confusing you is the fact that context freedom is different from ambiguity. This a simplified version of your C++ example:
a < b > (c)
This is ambiguous in that by looking at it alone you cannot tell whether this is a function template call or a boolean expression. The previous example on the other hand is not ambiguous; From the point of view of grammars it can only be interpreted as:
identifier assignment identifier binary-operator identifier semi-colon
In some cases you can resolve ambiguities by introducing context sensitivity at the grammar level. I don't think this is the case with the ambiguous example above: in this case you cannot eliminate the ambiguity without knowing whether a is a template or not. Note that when such information is not available, for instance when it depends on a specific template specialization, the language provides ways to resolve ambiguities: that is why you sometimes have to use typename to refer to certain types within templates or to use template when you call member function templates.
There are already a lot of good answers, but since you are uninformed about grammars, parsers and compilers etc, let me demonstrate this by an example.
First, the concept of grammars are quite intuitive. Imagine a set of rules:
S -> a T
T -> b G t
T -> Y d
b G -> a Y b
Y -> c
Y -> lambda (nothing)
And imagine you start with S. The capital letters are non-terminals and the small letters are terminals. This means that if you get a sentence of all terminals, you can say the grammar generated that sentence as a "word" in the language. Imagine such substitutions with the above grammar (The phrase between *phrase* is the one being replaced):
*S* -> a *T* -> a *b G* t -> a a *Y* b t -> a a b t
So, I could create aabt with this grammar.
Ok, back to main line.
Let us assume a simple language. You have numbers, two types (int and string) and variables. You can do multiplication on integers and addition on strings but not the other way around.
First thing you need, is a lexer. That is usually a regular grammar (or equal to it, a DFA, or equally a regular expression) that matches the program tokens. It is common to express them in regular expressions. In our example:
(I'm making these syntaxes up)
number: [1-9][0-9]* // One digit from 1 to 9, followed by any number
// of digits from 0-9
variable: [a-zA-Z_][a-zA-Z_0-9]* // You get the idea. First a-z or A-Z or _
// then as many a-z or A-Z or _ or 0-9
// this is similar to C
int: 'i' 'n' 't'
string: 's' 't' 'r' 'i' 'n' 'g'
equal: '='
plus: '+'
multiply: '*'
whitespace: (' ' or '\n' or '\t' or '\r')* // to ignore this type of token
So, now you got a regular grammar, tokenizing your input, but it understands nothing of the structure.
Then you need a parser. The parser, is usually a context free grammar. A context free grammar means, in the grammar you only have single nonterminals on the left side of grammar rules. In the example in the beginning of this answer, the rule
b G -> a Y b
makes the grammar context-sensitive because on the left you have b G and not just G. What does this mean?
Well, when you write a grammar, each of the nonterminals have a meaning. Let's write a context-free grammar for our example (| means or. As if writing many rules in the same line):
program -> statement program | lambda
statement -> declaration | executable
declaration -> int variable | string variable
executable -> variable equal expression
expression -> integer_type | string_type
integer_type -> variable multiply variable |
variable multiply number |
number multiply variable |
number multiply number
string_type -> variable plus variable
Now this grammar can accept this code:
x = 1*y
int x
string y
z = x+y
Grammatically, this code is correct. So, let's get back to what context-free means. As you can see in the example above, when you expand executable, you generate one statement of the form variable = operand operator operand without any consideration which part of code you are at. Whether the very beginning or middle, whether the variables are defined or not, or whether the types match, you don't know and you don't care.
Next, you need semantics. This is were context-sensitive grammars come into play. First, let me tell you that in reality, no one actually writes a context sensitive grammar (because parsing it is too difficult), but rather bit pieces of code that the parser calls when parsing the input (called action routines. Although this is not the only way). Formally, however, you can define all you need. For example, to make sure you define a variable before using it, instead of this
executable -> variable equal expression
you have to have something like:
declaration some_code executable -> declaration some_code variable equal expression
more complex though, to make sure the variable in declaration matches the one being calculated.
Anyway, I just wanted to give you the idea. So, all these things are context-sensitive:
Type checking
Number of arguments to function
default value to function
if member exists in obj in code: obj.member
Almost anything that's not like: missing ; or }
I hope you got an idea what are the differences (If you didn't, I'd be more than happy to explain).
So in summary:
Lexer uses a regular grammar to tokenize input
Parser uses a context-free grammar to make sure the program is in correct structure
Semantic analyzer uses a context-sensitive grammar to do type-checking, parameter matching etc etc
It is not necessarily always like that though. This just shows you how each level needs to get more powerful to be able to do more stuff. However, each of the mentioned compiler levels could in fact be more powerful.
For example, one language that I don't remember, used array subscription and function call both with parentheses and therefore it required the parser to go look up the type (context-sensitive related stuff) of the variable and determine which rule (function_call or array_substitution) to take.
If you design a language with lexer that has regular expressions that overlap, then you would need to also look up the context to determine which type of token you are matching.
To get to your question! With the example you mentioned, it is clear that the c++ grammar is not context-free. The language D, I have absolutely no idea, but you should be able to reason about it now. Think of it this way: In a context free grammar, a nonterminal can expand without taking into consideration anything, BUT the structure of the language. Similar to what you said, it expands, without "looking" anywhere else.
A familiar example would be natural languages. For example in English, you say:
sentence -> subject verb object clause
clause -> .... | lambda
Well, sentence and clause are nonterminals here. With this grammar you can create these sentences:
I go there because I want to
or
I jump you that I is air
As you can see, the second one has the correct structure, but is meaningless. As long as a context free grammar is concerned, the meaning doesn't matter. It just expands verb to whatever verb without "looking" at the rest of the sentence.
So if you think D has to at some point check how something was defined elsewhere, just to say the program is structurally correct, then its grammar is not context-free. If you isolate any part of the code and it still can say that it is structurally correct, then it is context-free.
There is a construct in D's lexer:
string ::= q" Delim1 Chars newline Delim2 "
where Delim1 and Delim2 are matching identifiers, and Chars does not contain newline Delim2.
This construct is context sensitive, therefore D's lexer grammar is context sensitive.
It's been a few years since I've worked with D's grammar much, so I can't remember all the trouble spots off the top of my head, or even if any of them make D's parser grammar context sensitive, but I believe they do not. From recall, I would say D's grammar is context free, not LL(k) for any k, and it has an obnoxious amount of ambiguity.
The grammar cannot be context-free if I need I can't tell the type of
an expression just by looking at it.
No, that's flat out wrong. The grammar cannot be context-free if you can't tell if it is an expression just by looking at it and the parser's current state (am I in a function, in a namespace, etc).
The type of an expression, however, is a semantic meaning, not syntactic, and the parser and the grammar do not give a penny about types or semantic validity or whether or not you can have tuples as values or keys in hashmaps, or if you defined that identifier before using it.
The grammar doesn't care what it means, or if that makes sense. It only cares about what it is.
To answer the question of if a programming language is context free you must first decide where to draw the line between syntax and semantics. As an extreme example, it is illegal in C for a program to use the value of some kinds of integers after they have been allowed to overflow. Clearly this can't be checked at compile time, let alone parse time:
void Fn() {
int i = INT_MAX;
FnThatMightNotReturn(); // halting problem?
i++;
if(Test(i)) printf("Weeee!\n");
}
As a less extreme example that others have pointed out, deceleration before use rules can't be enforced in a context free syntax so if you wish to keep your syntax pass context free, then that must be deferred to the next pass.
As a practical definition, I would start with the question of: Can you correctly and unambiguously determine the parse tree of all correct programs using a context free grammar and, for all incorrect programs (that the language requires be rejected), either reject them as syntactically invalid or produce a parse tree that the later passes can identify as invalid and reject?
Given that the most correct spec for the D syntax is a parser (IIRC an LL parser) I strongly suspect that it is in fact context free by the definition I suggested.
Note: the above says nothing about what grammar the language documentation or a given parser uses, only if a context free grammar exists. Also, the only full documentation on the D language is the source code of the compiler DMD.
These answers are making my head hurt.
First of all, the complications with low level languages and figuring out whether they are context-free or not, is that the language you write in is often processed in many steps.
In C++ (order may be off, but that shouldn't invalidate my point):
it has to process macros and other preprocessor stuffs
it has to interpret templates
it finally interprets your code.
Because the first step can change the context of the second step and the second step can change the context of the third step, the language YOU write in (including all of these steps) is context sensitive.
The reason people will try and defend a language (stating it is context-free) is, because the only exceptions that adds context are the traceable preprocessor statements and template calls. You only have to follow two restricted exceptions to the rules to pretend the language is context-free.
Most languages are context-sensitive overall, but most languages only have these minor exceptions to being context-free.