[Haskell-cafe] Solving the configuration problem with parametrized modules

Yitzchak Gale gale at sefer.org
Tue Sep 6 14:17:19 CEST 2011

Joachim Breitner wrote:
> ...Usually in Haskell, you’d determine them in the main function and then
> pass them in an argument to all functions needing them, or you wrap that
> in a monad...
> The big downside is the verbosity of the approach: A lot of parameters
> need to be passed, and if one such value is suddenly required in a
> function where it was not before, this function’s signature and all
> functions using it have to be modified.

I'm struggling to understand what problem you are trying to solve.

We're talking about passing a single parameter - a record type,
or a shallow tree, or something else extremely simple.
In the monadic case, we're adding a single Reader component
to the transformer stack.

I'm currently working on a large system, consisting of hundreds
of modules and tens of thousands of lines of code. There
are hundreds of configuration parameters, coming from
configuration files, modules of compiled-in constants,
and the UI. Different subsets of the parameters are needed
in various subsystems.

Doing that in Haskell has been an absolute pleasure, much much
simpler than in any other language I've used for a system
of this size. (I'll spare you the horror of what I had to do once
for a large system written in C++...) I barely have to think about
it - the types reflect the relationships between the configuration
and the various parts of the system, and Haskell's type system
does the rest automatically, guaranteeing correctness. What
else could you ask for?

> Also, I expect that the explicit
> passing causes a small performance penalty.

We're been processing gigabytes of data - doing non-trivial operations
including a kind of image recognition and more - so far with nearly
imperceptible delay. The bottleneck, if there is one, will certainly
not be the configuration subsystem.

I know some Haskell libraries use constructor hiding, or even
the data-default package which uses generics, to add more
flexibility to the configuration system. But I don't feel the
need even for those. I certainly wouldn't dream of dragging in
type-level olegery, unsafe coercion, implicit parameters
and other experimental extensions. Simplicity just works.

What case are you thinking about? A system several orders
of magnitude more complex? Some special case that creates


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