Open research problems

General problems

Efficiency of lazy functional languages

This is a problem that came up during IRC discussions. We consider a purely functional language L:

• By "purely functional" we mean a language that has value semantics; that is, there is no function such that after evaluation of the function the value that was referred to by something else changed. (Also known as "No Side Effects").
• A value is "changed" when it is not the case during an evaluation that when the old value and the new value would both be fully evaluated, there wouldn't be the same result. This should make sure that laziness is allowed in the purely functional language.
  

Can purely functional languages like L can implement all algorithms that can be implemented in a language like C as efficiently in an amortized sense, ignoring space-usage?

I/O

Some pertinent quotes:

This is hard stuff. Two years ago I spent several hours to write 3 lines invoking IO computations.

Trying to understand the IO (); "belka", Haskell Discourse.

Haskell compromises brilliantly, by fencing off pure and impure functions from one another [...] The illusion is so good that programmers are fooled into thinking I/O is pure in Haskell. And now that we can write mostly pure code with occasional impure wrappers, researchers have mostly stopped seeking superior alternatives.

A Problem With I/O, Ben Lynn.

Input/output is awkward in declarative languages. Some functional languages like LISP have procedural read and write operations. Prolog has ugly read and write "predicates" that execute in sequence. Haskell monads provide pure functional I/O but still involve a sequence of actions.

Specifying Input/Output by Enumeration, Walter W. Wilson and Yu Lei.

Once you’re in the IO monad, you’re stuck there forever, and are reduced to Algol-style imperative programming. You cannot easily convert between functional and monadic style without a radical restructuring of code.

Of Course ML Has Monads!, Robert Harper.

Functional programmers used to worry about how to solve “the I/O problem”. Functional programming (FP) eschews any notion of side-effect or order of execution, and instead has an order-independent notion of evaluation. In contrast, input & output (I/O) seems to be an inherently effectful, ordered notion.

[...]

In a sense, I see us as in a worse position than before. Since the adoption of monadic IO, it’s been less immediately apparent that we’re still enslaved [...] Our current home is not painful enough to goad us onward, as were continuation-based I/O and stream-based I/O. (See A History of Haskell, section 7.1.) Nor does it fully liberate us.

Can functional programming be liberated from the von Neumann paradigm?, Conal Elliott.

Sadly [...] many Haskell programmers believe that IO is necessary to do "real programming", and they use Haskell as if it were C (relegating lots of work to IO). In other words, monadic IO has proved to be such a comfortable "solution" to I/O in a functional language, that very few folks are still searching for a genuinely (not merely technically) functional solution. Before monadic IO, there was a lot of vibrant and imaginative work on functional I/O. [...]

The C language is purely functional, Conal Elliott.

Stream transformers are fragile to use, continuations are powerful but somewhat clutter the syntax of functions. Monads and uniqueness types both present a trade-off, do we accept the over-sequentialisation imposed by monads, or the visual disorder of explicit environment passing? We believe that a compromise is still to be found [...]

Approaches to Functional I/O, Owen Stephens.

The common method to relieve the programming language designer from the inherent IO-problems is to shift responsibility to the programmer who has to sequentialize all IO-requests. This is also true for the monadic approach implemented in Haskell.

FUNDIO: A Lambda-Calculus With letrec, case, Constructors, and an IO-Interface:, Manfred Schmidt-Schauß.

The programming style in a lazy functional language is heavily influenced by the supported I/O-mechanism. Modifying the I/O-behaviour or debugging some lazy functional program that uses I/O is a black art. It is interesting that novices in lazy functional programming in general expect that there is some direct (side-effecting) I/O using a function call.

A Partial Rehabilitation of Side-Effecting I/O:, Manfred Schmidt-Schauß.

The downside of I/O using monads is the need for a monad that can not be unwrapped. So, when using monadic I/O there is no way to get rid of the I/O monad. Furthermore, it is not as intuitive as one would like it to be. A prerequisite to good software design is a thorough understanding of the structures and glues of the implementation language. [...] Yet the understanding of monads is not trivial. The extensive amount of tutorials and questions on the Internet strengthen this thought.

Input/Output in Functional Languages (Using Algebraic Union Types), R.J. Rorije.

The notation for interactive programs written in the monadic style is irritatingly close to the notation used in imperative languages.
[...]
Uniqueness typing addresses the more general problem of statically controlled use of resources in functional programs and, even if combined with passing unique representations of environment objects as arguments to these programs, it does not suffice to solve the input/output-problem.
[...]
Similarly, monads are used to address the more general problem of computations (involving state, input/output, backtracking, ...) returning values: they do not solve any input/output-problems directly but rather provide an elegant and flexible abstraction of many solutions to related problems.
[...]
So, both input/output-schemes merely provide frameworks in which side-effecting operations can safely be used with a guaranteed order of execution and without affecting the properties of the purely functional parts of the language.

Functions, Frames and Interactions, Claus Reinke.

Although we all love the beautiful aspects of functional languages we must admit that it is difficult to deal with a beast called Input-Output (I/O).

The Beauty and the Beast, Peter Achten and Rinus Plasmeijer.

Questions:

• Is there an alternate standalone model of I/O with less problems than each of the current models?
• If not, can I/O be moved away from the language (as a model) and into the implementation (making the language denotative), while keeping the resulting language practical to use?
• Or would any denotative language be solipsistic?

Other articles:

• An alternative approach to I/O, Maarten Fokkinga and Jan Kuper.

• Witnessing Side Effects, Tachio Terauchi and Alex Aiken.

• Assignments for Applicative Languages, Vipin Swarup, Uday S. Reddy and Evan Ireland.

• Imperative Functional Programming, Uday S. Reddy.

• Functional Programming with Side Effects, Mark B. Josephs.

• Lambda Calculus For Engineers, Pieter H. Hartel and Willem G. Vree.

• On Zero-Side-Effect Interactive Programming, Actors, and FSMs, Sergey Ignatchenko.

• Unique Identifiers in Pure Functional Languages, Péter Diviánszky.

• Call-by-Need Is Clairvoyant Call-by-Value, Jennifer Hackett and Graham Hutton.

• Nondeterminism with Referential Transparency in Functional Programming Languages, F. Warren Burton.

Specific problems

Implement encapsulated-state interface entirely in Haskell (no primitives)

Implement Data.STRef and Control.Monad.ST.runST without using the built-in monadic ST or IO types. This needs to happen with operations that all run in O(1) amortized time.