On 2004-06-01T18:06:36+0200, George Russell wrote: > The most common use of unsafePerformIO, for me at least, is initialisation. > There *surely* must be a better way of doing this, but I haven't really > seen much discussion of the topic. Here is my back-of-the-envelope > suggestion for a new interface, can anyone do better?
Oleg and I summarize the existing solutions to the configuration problem
in Section 2 of our draft paper at
http://www.eecs.harvard.edu/~ccshan/prepose/
We then propose a new solution.
Implicit configuration -- or, type classes reflect the value of types
The _configuration problem_ is to propagate run-time preferences
throughout a program, allowing multiple concurrent configuration
sets to coexist safely under staticly guaranteed separation. This
problem is common in all software systems, but particularly acute in
Haskell, where currently the most popular solution relies on unsafe
operations and compiler pragmas.
We solve the configuration problem in Haskell using only widely
implemented and stable language features like the type class
system. In our approach, a term expression can refer to run-time
configuration parameters just as it refers to compile-time constants
in global scope. Besides supporting such intuitive term notation,
our solution also helps improve the program's performance by
transparently dispatching to specialized code at run-time. We can
propagate any type of configuration data -- numbers, strings, IO
actions, polymorphic functions, closures, and abstract data types.
The enabling technique behind our solution is to propagate values
via types (literally), with the help of polymorphic recursion and
higher-ranked polymorphism. The technique essentially emulates
local type-class instance declarations. Configuration parameters
are propagated throughout the code implicitly as part of type
inference rather than explicitly by the programmer.
We are thinking of submitting this paper to the Haskell workshop. Any
comments would be appreciated, especially before Friday. (:
Ken
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