On Wed, Aug 24, 2011 at 04:35:42PM +0400, dokondr wrote:

Hi, What is the Haskell way to compose functions in run-time? Depending on configuration parameters I need to be able to compose function in several ways without recompilation. When program starts it reads configuration parameters from a text file. For example, I have three functions, f1, f2, f3, each doing some string processing. I need to support two configurations of string processors :

if param1 then sp = f1 . f2 . f3 else sp = f1 . f3

I'd like to avoid 'if' somehow and instead use some declarative way to specify code to run in external configuration file. In other words I need some easy tools to create mini DSLs without all the efforts usually involved with implementing full-blown DSL.

A simple alternative to if would be: options = [ ("foo", f1 . f2 . f3) , ("bar", f1 . f3 )] and then "lookup param options". I don't know if this is what you're looking for, though. regards, iustin

Hi,

if param1    then sp = f1 . f2 . f3    else sp = f1 . f3

If you have many situations like these, i.e. where one or several components are conditional, you can define a function when True f = f when False _ = id And now you can define sp like this: sp = f1 . when param1 f2 . f3 Regards, Jonas On 24 August 2011 14:35, dokondr wrote:

Hi, What is the Haskell way to compose functions in run-time? Depending on configuration parameters I need to be able to compose function in several ways without recompilation. When program starts it reads configuration parameters from a text file. For example, I have three functions, f1, f2, f3,  each doing some string processing. I need to support two configurations of string processors :

if param1    then sp = f1 . f2 . f3    else sp = f1 . f3

I'd like to avoid 'if' somehow and instead use some declarative way to specify code to run in external configuration file. In other words I need some easy tools to create mini DSLs without all the efforts usually involved with implementing full-blown DSL.

Thanks, Dmitri

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