Language Extensions

Much maligned but key to Haskell vitality

Extensions are bad?

It is common to hear complaints about the menagerie of Haskell LANGUAGE extensions as listed at the top of Haskell source modules. Those complaints have been most strongly expressed by those new to the language in my experience, but I get the sense that many Haskellers feel uneasy about the situation, that it is all a bit untidy and suggestive of a chaotic evolution of the language.

I get it. If you are relatively new to such a technical language then it is going to be alarming top see a quite extensive list of all the things that, for the most part, you are not going to have the foggiest idea of what it looks like or how it works.

Extensions are great!

I think the Haskell language extensions are good because:

  • The extensions are good in themselves, all of them. There are few that I have never made use of and many that I make extensive use of. If we did not have them then I am convinced Haskell would be a much less expressive language.

  • The language extension promote development and experimentation, which has been an important aspect of Haskell from the start — part of the Haskell DNA, so as to speak.

We have a tension here. We are frightening the bejasus out of those new to the language (stupid, huh) but would otherwise stop the progression of the language (streams I/O anyone?).

Let’s explore the alternatives.

Alternative 1 : Just enable them

Just enabling them all would be easy enough (though we might have to ditch pr modify a few where they clash, but let’s not get too pedantic) and of course the new users would be happy. For a while.

The problem is that some of these extensions are really, really terrible and are only there as break class options for those that really, know what they are doing (trust me). So maybe we can do without those. But then we have a Goldilocks problem — which extensions are too hot (too unsafe to include) and which are too cold (can’t exclude these without breaking the language). Different people are going to have different opinions on this and it is going to be very difficult to find a consensus (to put it mildly). I have even essayed at a recent Haskell eXchange (in a social context) that ‘well, at least we could have OverloadedStrings’ only to be met with breath being sucked in ‘I don’t know about that …’.

Alternative 2 : Waiting for Godot

Alternatively we can just insist that the extensions are passed fit for use before being let loose on the community. This sounds nice in theory and looks neat and tidy, sparing the blushes in those discussions with advocates of programming languages unencumbered with a lot less legacy than Haskell, but in reality we would still most likely be stuck with Haskell 2010 with the promised Haskell 202X morphing into Haskell 203X. Programming language reports require an enormous effort to bring together and seem to not fit well with the way we work now.

But fear not, I have a cunning plan!

Haskell 2020

Why don’t we get the committee to vote on the extensions they want and when they are done with all of the voting raise a GHC issue requesting the new language be accommodated — the GHC patch would probably take all of a minute to write by a competent GHC developer. From start to end, Haskell 2020 could be ready in a morning (all the hard work being done already).

Some folks might say we need extensive work to get the documentation all up to report standard. This is just letting the best become the enemy of the good. Surely, at the cost of extending the process, we could ask that the GHC documentation be improved before certain extensions could be accepted, after which said documentation can be aggregated into an addendum to the 2010 report to make the 2020 report. We should not be too fussy about the levels of documentation either as otherwise the risk of the whole process stalling re-presents itself. After all Haskell code bases have been developed for many years with the current levels of documentation (or much worse indeed, as the literature has been improving of late).

Got an issue with any of this? Please share or drop me a line (see below).