Should we strive to only have one language in our development efforts?
Throughout much of the last decade the fashion in the enterprise software world has been to focus on one standard language for software development efforts. Many development organizations strive to do all their work in Java (or C#/VB).
The rationale for this is that developers find it hard to be proficient in more than one language. Sticking to a single language lowers the learning burden, particularly when hiring new people.
There's some truth to this, but also much that's missing. The programming environment is partly language, but also about languages and frameworks. Larger frameworks, Hibernate, Struts, ADO, present as much of a challenge to learn as a language even if you program them in a single host language. Often the difficulty of expressing what you need in the host language is sufficiently awkward that many frameworks resort to configuration files, which are effectively external DomainSpecificLanguages written in XML - which adds a jigger of 80 proof ugliness to them.
For many developers, the one-language notion is a sign of lack of professionalism. This is best exemplified by the Pragmatic Programmers' advice to learn a new language every year. The point here is that programming languages do affect the way you think about programming, and learning new languages can do a lot to help you think about solving problems in different ways. (It's important to learn languages that are quite different in order to get the benefit of this. Java and C# are too similar to count.)
I agree with prags' advice here, as in most things. But I also sympathize with the overhead of learning a new language. My personal scripting is pretty much all done with Ruby and I've been loath to any more than play with reasonable alternatives like Python, Groovy, or PowerShell. It's not that it's hard to use the alternatives, but with Ruby I know too much that I'd have to look up with the alternatives.
The important point here is that when I'm writing these scripts I'm not manipulating new abstractions. Much of what I do is fiddling with text, the file system, and hunks of XML or YAML. If I need to take on a sizable new abstraction, the cost of learning it as a library isn't really much less than the cost of learning a language to manipulate it. If I want to specify a directed graph structure for display, learning Graphviz's Dot language is hardly more work than learning a new Ruby library.
Using a DSL instead of a library can offer us better ways of manipulating our abstractions. This makes it easier to see what we've written and to reveal our intentions. An API is like declaring a vocabulary, a DSL adds a grammar which allows you to write coherent sentences.
This argument is strong for DSLs, but does it also apply to general purpose languages? If you are working in Java (or soon C#) does it make sense to use Ruby now it's available on your platform?
The last decade has seen the rise of memory managed C-based languages. People saw that, despite many years of skepticism, memory management makes life sufficiently better that it was worth stepping away from C and C++ in the enterprise world. A common platform and language also pulled people away from proprietary walled gardens like Powerbuilder and Delphi.
Now there is a similar question. Are modern scripting languages another step forwards? Do we prefer their well-chosen terseness? Time and time again I hear experienced Java and C# developers report they are more effective in Ruby - which is why I've been encouraging Ruby. It wouldn't surprise me if similar reports appear in the next few years about other languages too.
A decade ago I was talking to my old friend Tom Hadfield at OOPSLA 96. Java's rise was apparent and it was clear that Smalltalk's future was doomed. Despite my love of Smalltalk I was pretty sanguine. I felt that Java gave people enough of what they needed; while it wasn't quite as nice as Smalltalk it was enough of an improvement over C++, particularly with memory management, for me to be happy with it. Tom disagreed, he felt there was something fundamentally different about the expressiveness of Smalltalk, the way you could better capture the intention of what you were doing directly in your code - closing the gap between domain knowledge and programming.
In the intervening years I've come to the view that Tom was right after all. After several years in curly brace land, Ruby reminded me of what I was missing. There's a clarity to reading Ruby code that just makes it an easier medium to work with, despite the inferior tooling. I'm way more sympathetic to the Smalltalk holdouts than I felt then, even though I haven't felt inclined to open an image in anger for a long time.
So are we returning to the language cacophony of the late 80's and early 90's? I think we will see multiple languages blathering away, but there will be an important difference. In the late 80's it was hard to get languages to inter-operate closely. These days there's a lot of attention to making environments that allow different language to co-exist closely. Scripting languages have traditionally had an intimate relationship with C. There's much effort to inter-operation on the JVM and CLR platforms. Too much has been invested in libraries for a language to ignore them.
So my sense is that we will see multiple languages used in projects with people choosing a language for what it can do in the same way that people choose frameworks now. I agree with Neal that we are entering a period of Polyglot Programming.