Is Perl a compiled or an interpreted programming language?

Question

Is Perl compiled or interpreted?

Cortege

Answer 1

Well, that depends on what you mean by a compiled language. Maybe this is why googling did not bring forth a clear answer to your question.

One viewpoint is that compilation means compiling from a source code description to another, i.e. code generation.

If we accept these premises, then Perl 6 can be compiled and Perl 5 and older are interpreted languages.

Perl 6 is specifically compiled to Parrot bytecode. Perl 6 is therefore a properly compiled language, in the same way say, Java is.

Perl 5 and older parses the Perl source code to an internal list or tree, but I don't think it should be called a proper compiler, except maybe in a theoretical sense. It does not output any bytecode, assembly or real machine code usually associated with compilers. The parsing stage of Perl to check Perl syntax used to be called "compiling" the source. It is used to check the syntactical validity of a Perl source file without running it.

It is invoked as:

perl -c myprog.pl

But if you look at the help for Perl options, -c actually stands for "check".

-c                check syntax only (runs BEGIN and CHECK blocks)

^{(To further complicate things, Perl 5 had support for writing out internal bytecode but it was removed in version 5.10. Presumably because it was buggy, I don't know.)}

On the other hand, if you argue that compilation is the act of parsing a source tree into any other kind of representation, well, that parsing makes Perl a compiled language. Perl must completely parse a source file before it can start executing it. By this definition, any language which can start executing a source file immediately before parsing would be an interpreted language.

A third way to look at this is from how these words, "interpreted" and "compiled" are most often used by professionals in the field. I would bet good money that if a random subset of programmers were asked to choose "compiled" or "interpreted" when thinking of Perl, most would choose "interpreted". Not because of some theoretical argument over the nature of Perl, but because "compiled" usually invokes thoughts of "compiling", "linking", "object code" etc, while "interpreted" is taken to mean "write the code, try it". Right or wrong, that may be good to know when trying to determine if Perl is, truly, interpreted or in fact, compiled. You are going to run into many arguments on your quest.

Commination

so does your first sentence is only correct regarding Perl6? Does it make Perl5 interpreted only language? – Cortege

This isn’t really accurate. If Perl isn’t compiled, where does the parse tree come out of? You can’t get a parse tree without a parser — which is all compiling is. Perhaps if the question had talked about code-generation of native machine language, your answer would fit better. But it didn’t, for code generation is quite a different thing from compilation. – Liberalism

@tchrist, OTOH, then almost all languages are compiled, because almost all of them have parsers. – Commination

@Liberalism No, parsing to make a syntax tree is not compiling. If you dsiagree, then you must believe that there doesn't exist any interpreted Lisp, because Lisps read stuff like (let ((a b)) c) and turn it into a tree in memory. However, there is a difference between running that statement by walking the tree, and generating code. – Tarrasa

Compiling means we generate something which is self-contained: it does not require help from an interpreter's run-time support for doing things like creating and resolving local variables, calling functions, or checking types. These things are done by machine language. Or at least a pseudo-machine language which has nothing to do with the specific interpreter for that specific language. When your "machine language" has a LAMBDA opcode to capture a lexical environment and make a function, that's not really compiling; it's quasi-compiling. – Tarrasa

@Prof. Falken, Re "then almost all languages are compiled", Of course they are. It would be silly to write a fully interpreted programming language. Only shell-type languages are interpreted now. – Hesketh

Re "One viewpoint is that compilation means compiling from a source code description to another, i.e. code generation. If we accept these premises, then Perl 6 can be compiled and Perl 5 and older are interpreted languages", huh? By that definition, Perl 5 is compiled. It produces an assembler program composed of high-level opcodes. /// Re "It does not output any bytecode, assembly or real machine code usually associated with compilers", It does; it just doesn't bother writing it to disk. There was a tool to do so, but it wasn't maintained. So by both of your definitions, Perl5 is compiled – Hesketh

@Hesketh Well, at least PicoLisp is interpreted IIRC. I did not know that Perl 5 also uses bytecode. EDIT - Also look at my other paragraphs - I am not actually trying to argue one way or another. I maybe should rewrite the answer a bit, but not today. What I am roughly going for is an expose over arguments why Perl 5 may or may not be called a compiled language. In any case, Perl (5) seems to be a language that somehow is hard to get a consensus on. FORTRAN - compiled. PicoLisp - interpreted. Perl? Lots of arguing... – Commination

Re "Perl (5) seems to be a language that somehow is hard to get a consensus on.", Perl "explicitly executes stored precompiled code made by a compiler which is part of the interpreter system". That's a definition of an interpreter, so Perl is an interpreter that includes a compiler. The lack of concensus comes from people incorrectly assuming it has to be one or the other. – Hesketh

Answer 2

You aren’t going to get a definite answer, because you haven’t provided a definite question.

Perl is always in one of two states: it is either compiling, or it is executing. That’s why you see talk of “at compile-time” vs “at run-time”. Normally, you get one compile phrase followed by one execution phase, but it need not be that way.

These two phases can also trade back and forth. An eval STRING is a way for the interpreter to call the compiler (so too therefore are do FILE and require). A BEGIN block is a way for the compiler to call the interpreter (so too therefore are use and no).

When you run perl -c, you omit the run-time phase. There are various ways to skip the compile-time phase, but none of them is particularly convenient, or commonplace. Apache’s mod_perl only compiles scripts once but executes them many times. If you use the Byteloader, you can do the same. Et cetera.

The correct answer to whether Perl is compiled or interpreted is simply YES.

Liberalism

Wouldn't the same be said of any interpreted language, python as a case in point? – Atalya

Answer 3

Both. Perl5 compiles the source code into OPCODE objects, then interprets the OPCODE objects. Long answer follows.

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From Wikipedia,

A compiler is a computer program (or set of programs) that transforms source code written in a programming language (the source language) into another computer language (the target language, often having a binary form known as object code).

Perl5 is a compiler. It takes Perl5 source code and produces of a graph of OPCODE objects.

$ perl -MO=Concise,-exec -E'for (1..3) { say "Hello, World!" }'
1  <0> enter 
2  <;> nextstate(main 48 -e:1) v:%,2048
3  <0> pushmark s
4  <$> const(IV 1) s
5  <$> const(IV 3) s
6  <$> gv(*_) s
7  <{> enteriter(next->c last->f redo->8) lKS/8
d  <0> iter s
e  <|> and(other->8) vK/1
8      <;> nextstate(main 47 -e:1) v:%,2048
9      <0> pushmark s
a      <$> const(PV "Hello, World!") s
b      <@> say vK
c      <0> unstack v
           goto d
f  <2> leaveloop vK/2
g  <@> leave[1 ref] vKP/REFC
-e syntax OK

However, the Perl5 compiler does not produce machine code. So how is the OPCODE graph executed? From Wikipedia, one definition for an interpreter is something that

explicitly executes stored precompiled code made by a compiler which is part of the interpreter system

This means the OPCODE graphs is interpreted.

Work was being done to provide the option to compile Perl5 to LLVM bytecode. This, in turn, can be jit compiled into machine code. This is the same approach Java uses.

Hesketh

Professionals use the definition given in this answer. Plebes tend to use something murkier. – Liberalism

@tchrist, I would not expect a professional to discuss languages in terms of whether they are interpreted or compiled. – Hesketh

@Hesketh I would. There are semantic differences. Programs in interpreted languages can do things which are impossible to compile. If such use is predominant among the users of a language, then it is strictly an interpreted language. For instance, an interpreted language can allow user-defined programs to extend the interpreter with custom interpretation routines. No compiler can determine what these routines do (or whether they even halt for a given set of inputs) to be able to emit the equivalent code. – Tarrasa

@Hesketh Hmm, try googling for "fexpr"; that might be better. – Tarrasa

@Hesketh The issue is not whether the body of the fexpr is compiled or not, but whether the actions of the fexpr can be replaced by code generation, so the fexpr is no longer used. I.e. the compiler looks at the code and sees, "Aha, this is calling a loop fexpr. Judging by the code of that fexpr, it's a loop. So I will just generate a loop that doesn't call the fexpr any more". If you can compile user-defined fexprs in the language itself, that makes fexprs unnecessary. Why optimize interpreters, when you're already compiling? – Tarrasa

@Kaz, Sorry, I have no idea what you just said, and more importantly, I still don't see how fexprs lead professionals to discuss languages in term of whether they are interpreted or compiled. – Hesketh

Answer 4

Raku (Perl 6) has compatibility mode with Perl 5, so Perl 5 can be compiled to JVM and Parrot using Rakudo. I want LLVM for this!

Rella

Answer 5

Perl is an interpreted language. However, it does compile internally into p-code for efficiency.

Hsu

I don't think what is usually called p-code ( en.wikipedia.org/wiki/P-code_machine ) can be compared to what Perl is usually parsed into. – Commination

p-code is like ByteCode. P-Code-Machine is like JVM. Actual structure of translated state depends on Perl implementation. BTW, there is an implementation of Perl for JVM too : javainc.com/projects/jperl similarly there are implementations that use ByteCode of CLR. – Hsu

Ah, I see now about the ByteCode. Thanks. Interesting about jperl. But it seems it just interprets Perl? It would have been cooler (but harder I guess) if it compiled Perl to class files. – Commination

Perl5 produces a graph of opcode objects. What it produces is not p-code as it is neither portable nor instructions for some virtual machine. – Hesketh

Answer 6

From Wikipedia: "Perl is a high-level, general-purpose, interpreted, dynamic programming language". Perl 6 allows also for compilation (again, see Wikipedia).

Zettazeugma

Thank you for the only straightforward answer on the damn page. – Dorwin

Answer 7

Both. First, Perl 6 script is compiled to bytecode (and optimized). Then it is executed (however, you still need Perl interpreter for this). Bytecode is kind of executable code, which is independent from environment it runs on (the same bytecode can run on Unix enviroment on ARM processor, Windows system with x86 and Haiku on x64).

Perl 6 can be compiled to Parrot VM (Virtual Machine) bytecode. Parrot VM is used by Python and Ruby as well.

This is what makes Perl, Ruby and Python faster than PHP, which is just interpreted (can be compiled as well, but you need 3rd party components for this).

Plebe

Thought of Perl 6. Sorry, will fix it. – Plebe

I retract that, seems it is compiled to some sort of bytecode, although official support for writing bytecode to files was removed at 5.10. – Commination

I wouldn't really call it a bytecode. These text may explain it: socialtext.net/perl5/optree_guts perldoc.perl.org/perlguts.html#Compiled-code – Plebe

@Amigable Clark Kant, You are refering to perlcc. perlcc attempted to serialize Perl5's opcodes. It was never part of the process of executing Perl5 code. – Hesketh

Don’t use the past tense when describing perlcc. It was simply de-coupled from the core. It was not retired. There’s been a lot of work done on it. – Liberalism

@tchrist, I was under the impression that it didn't even run against newer version of Perl. Are you saying I remember incorrectly (entirely possible) or that it isn't the case anymore? (PS - I don't get notified if you don't tag me.) – Hesketh

Answer 8

It's a VM that can also be compiled without the need of C. Its VM however is light years faster than Java so there really isn't anything that compares to it.

The bad - Its terrible in the same way C++ pushes to be a be all end all swiss army knife. In Perls case it removes the training wheels, safety and borders. For example in most OOP languages you must declare a class before you can use an object, in Perl there are no restrictions on types you can mix and match anything thereby turning anything into an object or an object into anything. The concept is confusing as hell if you already know OOP, the lack of restrictions is both powerful and a drawback in Perl. So whether the language is compiled or not isn't the issue so much as can you actually reason something effectively without braking the world. Another issue is that Perl is write only thus everything looks like gibberish once written and becomes difficult to dissect and even debug.

Barathea

Answer 9

Most often interpreted but may be compiled also. about perl compiler

Strut

can you define 'most often' :) – Cortege