Hi,
Sorry for writing in english, I am not from France.
Take a look here, please:
http://www.haskell.org/haskellwiki/Modular_Monadic_Compilers_for_Programming...
The source at #3) on the above page is a code generator written using the State monad.
The state monad is necessary because here it provides a sort of context, like a set of the global variables
in imperative programming.
And that is why we are using The State Monad, to simulate the
use of a set of global (or local) variables, used to store values.
Especially in this example, the lenghts of the pieces of generated code are stored in order to compute the length of the biggers codes, when they are catenated, (glued together).
Other monads have specific use. Parser monad is used for glueing parsers, the list monad can simulate paralel computations and even backtracking (there is a backtracking monad too), the Maybe monad is used to make computations including Nothing in the set of values, etc.
Basically monadic capsules can be seen like Christmas Giftts containing compuations. And monadic operators have two kind of use:
- return is a package maker, it creates a capsule, a package, like wrapping a gift
- bind is something like: how to combine a function with a package. Open the package, compose functions, take care to produce an other package (or capsule).
Hoping it helps,
Sincerely yours,
Dan Popa
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