On Tuesday, 2003-08-19, 12:42, CEST, Konrad Hinsen wrote:
I have been following the recent "Monad tutorial" discussion with interest, and even read the tutorial, which is a useful addition to the existing Haskell documentation. So useful in fact that it raises a question...
The whole monad mechanism seems to geared towards functions of one argument, plus eventually state, that get chained together. How about functions with several arguments?
As an example, I'll use the Maybe monad. Suppose I want to write code to handle experimental data, in which there might be missing values. I might then decide to represent measurements by data of type "Maybe Double", with missing values represented by "Nothing". I could then go on to define functions on missing values, which would return "Nothing" when their argument is "Nothing", and I could string these functions together via the monad mechanism. Fine. But how would I handle e.g. addition of two such values? The result should be "Nothing" when either of its arguments is "Nothing". Is there any mechanism to handle that?
Yes, liftM2. Defined in module Monad (or Data.Monad resp.).
Konrad.
Wolfgang