On 07/28/2012 03:35 PM, Thiago Negri wrote:
[...] As Monads are used for sequencing, first thing I did was to define the following data type:
data TableDefinition a = Match a a (TableDefinition a) | Restart
So TableDefinition a is like [(a, a)].
[...]
So, to create a replacement table:
table' :: TableDefinition Char table' = Match 'a' 'b' (Match 'A' 'B' Restart)
It look like a Monad (for me), as I can sequence any number of replacement values:
table'' :: TableDefinition Char table'' = Match 'a' 'c' (Match 'c' 'a' (Match 'b' 'e' (Match 'e' 'b' Restart)))
Yes, but monads aren't just about sequencing. I like to see a monad as a generalized computation (e.g. nondeterministic, involving IO, involving state etc). Therefore, you should ask yourself if TableDefinition can be seen as some kind of abstract "computation". In particular, can you "execute" a computation and "extract" its result? as in do r <- Match 'a' 'c' Restart if r == 'y' then Restart else Match 2 3 (Match 3 4 Restart) Doesn't immediately make sense to me. In particular think about the different possible result types of a TableDefinition computation. If all you want is sequencing, you might be looking for a Monoid instance instead, corresponding to the Monoid instance of [b], where b=(a,a) here.
[...]
I'd like to define the same data structure as:
newTable :: TableDefinition Char newTable = do 'a' :> 'b' 'A' :> 'B'
But I can't figure a way to define a Monad instance for that. :(
The desugaring of the example looks like this: ('a' :> 'b') >> ('A' :> 'B') Only (>>) is used, but not (>>=) (i.e. results are always discarded). If this is the only case that makes sense, you're probably looking for a Monoid instead (see above) -- Steffen