This article is a list, in no particular order, of features I want to see in programming languages. I note practical use cases, and the extent to which these features affect language complexity.

# Units of Measure

Type systems allow us to catch errors along the lines of trying to push a square peg into a round hole: adding integers to strings or calling map on an integer. A units of measure system allows us to further annotate our values with, well, units.

Ideally, the language would allow us to specify both magnitudes – such as mass, time and velocity – and units belonging to that magnitude – grams, pounds both measure mass. But even without that, the compiler should provide these guarantees:

1. Naturally, the compiler checks operations for dimensional validity: you can’t add a time-annotated value to a velocity-annotated value without compilation failing.
2. Dimensional analysis is automatically performed: dividing a value representing distance over one representing time, the resulting value should be annotated with a velocity unit. Ideally, the language would allow units and magnitudes to be defined in terms of arithmetic operators composing others, so the compiler can simplify the units in complex computations to their simplest, human-readable representation.
3. Whether automated conversion between different units (e.g., adding Celsius to Fahrenheit would yield a value converted to the obviously correct and canonical choice of degrees Kelvin) should be performed by the language is a matter of taste and depends on whether the language designers’ place emphasis on correctness or ease of use.

As a brief aside, this is what the infamous Hungarian notation set out to correct: as Joel Spolsky noted, the original intent was to prefix variables with a short string denoting, not their type, but their semantic intent. In the example he quotes, integer variables are tagged with their coordinate system.

Admittedly, phantom types give us 90% of type-level semantic distinction with 10% of the work involved in bolting units of measure into a language. I still believe that a units of measure system like the one found in F#, where dimensional analysis is automatically carried out by the compiler, is a worthwhile addition to any language that emphasizes compile-time correctness.

In terms of language complexity, units of measure can be entirely orthogonal to the rest of the language, being unrelated to – but “parallel to”, for lack of a better term – type information. Typechecking can be implemented while maintaining total ignorance over the units of measure, with unit checking and dimensional analysis being carried out in a later stage of compilation. All unit-related code (conversion etc.) can safely be converted to arithmetic operations so code further down the compiler pipeline doesn’t need to be updated to take unit information into account.

In others, a units of measure system doesn’t increase language or implementation complexity.

# Effects

Most programming languages, with the exception of languages using monads for mutation like Haskell, have an “everything is allowed” model of effects. Logging calls, memory allocation, file access, etc. can be performed from just about anywhere. An effects system allows the programmer to assign permissions to volumes of their code: these parts can’t allocate memory – or, consequently, call parts that allocate memory – and must run in constant space, this part can’t talk over the network, this callback must be a pure function, etc.

What does an effects system do, exactly?

1. Functions – or lexical blocks, for increased granularity – are annotated with the side-effects they will carry out, or won’t (for example, we can annotate a function as being banned from using the standard input/output streams, a pervasive side-effect). The compiler determines the function’s actual effects signature by looking at its body: a function’s effects are the union of the effects of all its forms. The base case of this recursion is that some language built-ins will have side effects (new in C++ allocates, open in Python interacts with the filesystem), and also that FFI declarations must allow some way to claim “this foreign function, whose definition is inaccessible to you, has such and such effects”.

This allows the compiler to know the difference between what the code actually does, and what it should and shouldn’t do, and treat the difference as an error.

2. The function’s declared effects signature, if any, is checked against its actual effects: if we allocate memory, or call a function that allocates memory, and so forth, in a function which we have sworn to the compiler allocates nothing, then the compiler signals an error. (An obvious problem arises here: what if we need to add a debugging printf in a function we’ve declared as being free of IO? Or which is called by a function, somewhere up the call tree, that is declared as being IO-free? The debugging facilities would presumably provide a escape hatch from strict effect system enforcement.)
3. Calls to higher order functions are checked for effects-correctness. Just as a type checker prevents you from adding integers to strings, an effects system might prevent you from passing an impure function as the function argument of map – making map automatically and safely parallelizable, without requiring the entire language to be pure. A useful tool.

To elaborate upon #3, consider a sorting function, whose arguments are a sequence of T and a comparer function T -> T -> Ordering, where Ordering is just an enum of equal, greater than and less than, as in Haskell.

An effects system would allow us to annotate the comparer function with, for instance, a declaration that it should not allocate memory. So, only functions that don’t allocate can be passed as arguments to the sorting function. If the sorting function doesn’t allocate memory, then this allows us to make precise guarantees about the time-boundedness of the sorting function as a whole1. Other effects are also useful: annotating the function as being pure would rule out a large category of behaviour inside of it, possibly enabling better optimization.

Another use case would be for a library like libdill, which implements Go-style concurrency in C. Briefly, the function that takes a thunk and spawns a thread for it, using an effects system, could be annotated to only allow pure thunks to be passed to it, thus avoiding a whole category of concurrency bugs without any special infrastructure (see Rust), with the obvious downsides of only being able to spawn purely-functional threads.

Do effect systems increase language or implementation complexity? As with the case of units of measure, I don’t think so. Obviously this depends on how exactly you want them to work. A function’s effects declaration can be totally independent of its type signature, effects checking is also completely independent from type checking, like units of measure.

Unlike units of measure, an effects system would not be a curiosity to be brought up only on rare circumstances: side effects are everywhere and nearly everywhere they are implicit, documented in comments and docstrings, or not accounted for at all. An effects system makes them visible.

# Extensible Iteration

This is something I am less certain about. I’ve always looked at libraries like Iterate and For as examples of what a good macro system allows you to do – not to mention Common Lisp’s built-in Swiss Army knife iteration construct, the loop macro.

Still, I am not sure macro-based extensible iteration is the right approach for a language having a sophisticated type system. Maybe there’s a sweet spot, a system having both a type-centric extensibility mechanism, plus macros to make it look more like a part of the language itself, more like a DSL.

I don’t know what this would look like – combining the terseness and obviousness of loop code with a type-driven iterator mechanism – but I would like to explore this in the future. I am certain something worthwhile hides here.

# Refinement Types

Refinement types allow us to annotate terms with predicates, e.g., an integer term with n != 0 or a collection with a ‘non-empty’ tag. Functions can be annotated with refinement predicates – the obvious example is division requiring the denominator be non-zero – and their calls are checked automatically – that is, every division must be in a branch where the denominator can be proved to be non-zero.

This is obviously useful, but less straightforward to implement than either units of measure and an effects system. Additionally, unlike any of the previous features I mentioned, refinement types are not orthogonal to the type system but a fundamental change to how it works. You can’t bolt this onto an existing language, at least not without almost certainly losing important guarantees of your typechecking algorithm, like being able to complete in finite time). So any language that implements this universally useful feature will almost certainly have to be designed from the ground up, or be a feature-focused fork of a language like Rust.