What every programmer should ask when designing a new language

Here are some questions a prospective language designer should ask

himself when starting the designing a programming language.

– What need are you trying to fill? Don’t fall into the trap of “a

scripting language”, because they always turn into general-purpose


– What’s the metaphor? Even though you might not be trying to build a

“pure” language, it’s worth having a model for the core language,

such as “imperative, block-structured” (C), “object oriented”

(Smalltalk), “generic object orientation” (Lisp), “functional” (ML),

“lazy” (Haskell), “logic” (Prolog), “production system” (OPS5), etc.

These different core models influence the “natural” styles of program

development in different languages even if the set of available

facilities is similar. They also help define which late-arriving

features will “fit” and which will be warts.

[Jerry Jackson]

– How many programming paradigms does your language support? How tightly

are they integrated? Which other paradigms can you integrate with the

built-in facilities? How natural is the syntax of user-defined

extensions? Many problems are much better suited to some non-standard

programming model than to the usual object-oriented/functional

approaches. For example, constraint languages allow a very concise

description (and efficient solution) of many optimization problems.

Dylan supports functional and object-oriented programming in a tightly

integrated manner, but it offers no support for non-deterministic

programming, constraint-solving, etc., and not much support to add them

to the language. If you have first-class continuations in the language

you can add one additional programming model that requires non-standard

control flow, but, in general, different extensions based on call/cc

don’t work together.

[Matthias Holzl]

– Is high performance an issue? This says something about whether you

want to implement an interpreted, a VM-based, or a natively compiled


– Is high programmer productivity an issue? How important is this with

respect to performance? This decision can affect how you store

values and do function calling.

– How portable across platforms do you want the language to be? This

will relates to whether you want to compile to a VM or to machine

code, and to how well you support native libraries. It will also

affect library design for such things as graphics and GUI tools.

[Anton van Straaten]

– Do you want easily distributed executable code, i.e., do you you want

to allow code to be easily transmitted across networks and run

elsewhere, as Java does? Do you want to provide built-in support for

remote execution, like RPC/CORBA/RMI? If you are writing for a VM,

this can simplify some of these issue considerably.

[Anton van Straaten]

– What about debuggability? If you plan to compile it, you need to

think about how to store debugging information.

– How do you want to bootstrap it? This, too, says something about

what kind of back-end you might build. Perhaps you build a tiny VM

in C, then compile to C. This way, you avoid fun but time-consuming

work on code generation for modern super-scalar hardware, register

allocation, etc.

– Do you want to be able to catch type errors early or late? That says

something about your type system (whether you require that all types

be statically declared at compile-time, or allow them to be dynamic,

or have a hybrid scheme like Dylan does). In addition to the obvious

effect on performance, this decision will affects your memory model

in that completely static systems do not require tags or boxing.

– Will variables be associated with explicit type declarations?

– If yes, will these type declarations be required or optional?

– If optional, will the language use inferencing to supply unspecified

types, or simply use an all-purpose type (like Object or ‘any’)?

[Anton van Straaten]

– Will the language have any run-time type discrimination/checking at

all, or will types be completely statically determined? Some

languages considered statically typed still do some run-time checking,

such as Java.

[Anton van Straaten]

– Will any type checking happen at compile-time? Some languages with

explicit type declarations don’t always check types at compile-time,

such as old Visual Basic.

[Anton van Straaten]

– If you allow type declarations, you will want to think about whether

you want parameterized types. If you go whole hog with, say, F-bounded

polymorphism, you can get performance *and* type safety *and* ease of

use, but it’s hard to get this exactly right.

– What about namespaces? Do you want to have a simple scheme as in

Java, where classes, namespaces, and files are roughly equivalent?

Lisp-style packages? Dylan-style modules and libraries? Within a

single first-class namespace, how many second-class namespaces are

there? Java has 7 or 8: class names, function names, local variable

names, slot names, etc. Common Lisp has at least 3 (function,

variable, and class names). Dylan and Scheme have one, which greatly

simplifies things at a small loss of generality which can usually be

worked around with name conventions.

– What about encapsulation? Do you want to do information-hiding on a

per-class basis as in C++ and Java, or on a “module” basis as in Dylan?

– Is your language a functional language (that is, without side-effects)?

If so, is it an almost-functional language or a true pure functional

language? Or is there a functional core with some sort of machinery

for isolating side-effects, like monads do in Haskell?

– What kind of evaluation semantics does the language have? Eager as

in most languages, or lazy as in Haskell?

– Is your language purely lexical or do you offer dynamic variables (or,

more generally, access to the dynamic environment) as well? Dynamic

binding allows you to introduce local state for the duration of a

computation without side effects and without adding additional


[Matthias Holzl]

– Are there different semantics for “pointer-ish” and “non-pointer-ish”

values, like in C? Or is everything a first-class object reference,

like in Lisp? Having multiple ways of referencing values can make the

user mode much more complicated. On the other hand, making everything

be object references can require boxing and/or tagging schemes that

make your compiler and FFI more complex.

– How do you want to pass arguments to functions? By name as in Algol?

By value or by reference as in C? By object reference like Lisp does?

Is there more than one convention in the language?

– Do you want first-class functions? What about lexical closures?

First-class continuations? The answer to those questions will tell

you things about heap- and stack-allocation, and will also tell you

how important it might be to do a continuation-based compiler. It

also tells you how hard your compiler has to work to avoid consing

environments unnecessarily. Lots of sophisticated language designers

go with simple closures and avoid full continuations, because

full-scale environment capture is hard to do well.

– Does your language have an unwind-protect like facility? When you

design a new language it is tempting to include call/cc because it

allows you to do define many common (and uncommon) control structures.

On the other hand you want to have a facility that allows you to

reliably relinquish resources after you are done. If you simply try to

combine call/cc and unwind-protect, you immediately get the

“impenetrable shield vs. unstoppable force” problem in your language.

Possible solutions include: no call/cc, weakened unwind-protect,

different semantics for call/cc.

[Matthias Holzl]

– How do you handle conditions/errors? Return codes or signalling? Do

you have an unwinding-only model like C++/Java or do you allow restarts

like Dylan/CL? If you do the latter do you separate conditions and

restarts like Common Lisp or do unify them like Dylan? These questions

are important, because every programming language has to deal with error

conditions, and in many cases the unwinding model is used simply because

the language designer is not aware of any other possibilities.

[Matthias Holzl]

– Do you want the language to be “object-oriented” at all, given a

broad definition of OO that includes the spectrum from single

inheritance single receiver languages as in Java to multiple

inheritance multiple receiver languages as in CLOS? Do you want

to provide genericity through some sort of template scheme?

Here is how Jonathan Rees has characterized the very fuzzy term “OO”.

1. Encapsulation — the ability to hide the implementation of a type

2. Protection — the inability of the client of a type to detect its

implementation, guaranteeing that any changes to an implementation

that preserve the behavior of the interface will not break any

clients. This also gives some measure of “security”, because things

like passwords can’t leak out.

3. Ad hoc polymorphism — functions and data structures with parameters

that can take on values of many different types.

4. Parametric polymorphism — functions and data structures that

parameterize over arbitrary values, such as “a list of anything”). ML

and Lisp both have this. Java doesn’t quite because of its non-Object

primitive types.

5. Everything is an object — all values are objects. True in Dylan, but

not in Java because of its primitive types.

6. “All you can do is send a message” (AYCDISAM) = Actors model — there

is no direct manipulation of objects, only communication with (or

invocation of) them. The presence of fields in Java violates this.

7. Specification inheritance = subtyping — there are distinct types

known to the language with the property that a value of one type is as

good as a value of another for the purposes of type correctness. An

example is Java interface inheritance.

8. Implementation inheritance/reuse — having written one pile of code, a

similar pile (such as a superset) can be generated in a controlled

manner, that is the code doesn’t have to be copied and edited. A

limited and peculiar kind of abstraction. (E.g. Java class


9. Sum-of-product-of-function pattern — objects are, in effect,

restricted to be functions that take as first argument a distinguished

method key argument that is drawn from a finite set of simple names.

Some people say Lisp is OO, meaning {3,4,5,7}. Some people say Java is

OO, meaning {1,2,3,7,8,9}. E is supposed to be more OO than Java

because it has {1,2,3,4,5,7,9} and almost has 6; 8 (subclassing) is seen

as antagonistic to E’s goals and not necessary for OO. The conventional

Simula 67-like pattern of class and instance will get you {1,3,7,9},

which many people take as a definition for OO.

[Jonathan Rees]

– If the language is object-oriented, do you want it to be class-based

or prototype-based?

[Steve Dekorte]

– If you’ve got an object system, do you want it to have first-class

objects that exist in the run-time? Should the object system extend

to include all the way to the primitive types, or do you want to

special-case those like Java does? Do you want a Smalltalk/Java-style

single receiver object orientation, or a CLOS-style multi-method

generic function dispatch? If the former, do you need some sort of

static overloading like C++ has? If the latter and performance is

important, do you need some sort of Dylan-style “sealing” so that you

can do some compile-time optimizations? Do you want single

inheritance, single inheritance with interfaces, multiple

inheritance, or a hybrid single inheritance with mixins? If you’ve

got a more static type system, you’ll need to deal with casts. Do

you additionally want auto-conversion?

– If you’ve got an object system, how much of a meta-object system do

you want to expose? Do you want it to be purely reflective, or more

than that? In Dylan, we separated ‘make’ from ‘initialize’, which

was a good idea, but do you also want to separate out ‘allocate’, so

that you have control over where an object is created, e.g., in a

“persistent memory” pool that might be back-ended by a database?

– Do you need hairy CLOS-style method combination, or is a simpler

style like we did in Dylan enough? Do you care about what Gregor

Kiczales calls “aspects”, which might change your decision?

– A more general question that relates to the object system, the

meta-object system, and a different dimension of the bootstrapping

question is: do you want to implement a language which provides a

bunch of predefined and fixed constructs (such as an object system)

or do you want to provide a layered language that implements such

constructs in terms of lower-level features in the language? The

former is probably easier, but the latter can allow very flexible

customization, which tends to be traded off against standardization.

Note that even a language with a powerful built-in meta-object system

won’t necessarily allow you to replace that object system with

something else, for example, unless the language supports that sort

of thing.

[Anton van Straaten]

– How do you want to do memory management, manual or automatic (GC)?

– Do you want to support threading? Do you want to roll your own

threads or use OS threads? Do you want to support massive

concurrency like Erlang does? The answers to those questions will

tell you about aspects of the run-time, memory allocation/GC, and

performance. Oh yeah — it also tells you if you can actually take

advantage of the multiple processors sitting in most of the machines

we all have. Do you want Java-style synchronization where it is

built in to objects, or should that be handled orthogonally?

– If you have threads and continuations, how do they relate to each


– How well do you want to be able to integrate with native libraries?

This decision affects your memory model, how you plan to represent

run-time type info, how function call/return works, how signalling

works, etc. By “memory model”, I also mean to include what sorts of

objects are boxed or tagged. (Opinion: the Harlqn/FunO Dylan

compiler got it wrong — I think we should have boxed everything, and

then concentrated our efforts on box/unbox optimizations. This would

have *hugely* simplified FFI issues.) Good integration with native

code probably means that you will end up using a conservative

collector, and that will effect the semantics of “finalization” (if

you have it).

– Do you want to be able to return multiple values? How about &rest

arguments? These affect function call/return, tail-call elimination,

and stack vs. heap allocation optimizations.

– What’s your order of evaluation in expressions? This affects what

sort of optimizations can be safely done.

– What compilation model do you want? Lots of include files like

C[++]? Lots of “packages” like Java? Whole-worlds like Lisp?

Separate libraries like Dylan? This affects a lot of things, not

least of which is the ability to deliver small applications. It also

informs the design of your core run-time.

– Is the core run-time tiny like Scheme’s? Small like Dylan’s? Huge

like Common Lisp’s? If you like the Common Lisp model, it’s worth

looking at EuLisp to see how to re-package it in a more layered way.

– Even in a small run-time, you need to get the basic types right. Are

your numeric types “closed” (that is, do they include reals —

rationals and irrationals — and complex numbers)? Are your string

and character types rich enough to model Unicode?

– Think hard about collections. How do the following relate to each

other: sets, tables, vectors, arrays, lists, sequences, ranges? In

Dylan, we decided too late having the tail of a list be a “cons” was

maybe not such a great idea; what about that? How do your

collections interact with your threading model?

– Think hard about iteration, especially over collections. If all

collections obey a uniform iteration protocol, it means that you can

do things like ‘for e in c …’. Note that if iterators are done in

a first-class way, this has performance implications that your

compiler needs to worry about.

– Do you want some sort of security model built into the language?

What sort of model do you want to use? A simple “checker” like the

Java VM uses, or a more sophisticated capability-based model.

– What syntax do you want? Parentheses unaccountably give lots of

people hives, but S-expressions make a lot of things much simpler.

Infix syntax is quite nice when it’s done well, but you’ve got to get

the “kernel” of that exactly right if you want your infix macro

system ever to be usable. If you decide on S-expressions, should

they be represented as lists and conses, or do you want a first-class

object for that?

– Do you want to allow syntactic extensions (macros)? Lisp-style

macros? Dylan-style pattern-matching non-procedural hygienic macros?

Scheme-style ‘syntax-case’ pattern-matching procedural hygienic

macros? This says a lot about the syntax of your language, and it

also says a lot about the model you choose for compile-time

evaluation environments.

Thanks to Robert Eaglestone

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