To take this a step further, there is the DSL:
get :: m S
put :: S -> m ()
and the concrete implementation
m = State S
Of course, there are other monads which implement this DSL as well:
m = StateT S IO
m = Prompt StatePrompt
with
data StatePrompt a where
Get :: StatePrompt S
Put :: S -> StatePrompt ()
The "Prompt" solution(1) encodes your program (object of type Prompt
StatePrompt a) into a way such that it can be used with *any*
interpreter, whether it is the (s -> (a,s)) of State, or lifted into
part of some larger DSL, or whatever. For example, to lift into
StateT S IO:
interpretPrompt :: Prompt StatePrompt a -> StateT S IO a
interpretPrompt = runPromptM f where
f :: StatePrompt a -> StateT S IO a
f Get = get
f (Put x) = put x
So, I think a better way to describe it is that a DSL could be
implemented by many monads, and some monads let you interpret the DSL
into another monad. Even StateT is this way, in a sense; you
interpret it with runStateT:
runStateT myProgram initialState :: IO (S, a)
Now you have another program in a different monad (IO) which you need
to interpret somehow.
-- ryan
(1) http://hackage.haskell.org/package/MonadPrompt
2010/2/25 David Leimbach
Monads aren't necessarily EDSLs by themselves but are often shipped with functions that provide what would make them an EDSL. Take the State monad, it has at least a get and a put function to work with the state in the monad. That get and put are commands that function only within the domain of the State monad, and therefore could be thought of as an embedded language that is used to work with the state encapsulated in the State Monad computation. The way I like to think of it, a Monad provides an environment or a context within which it is very convenient to express an EDSL, and that this style of coding should be encouraged! :-) Dave 2010/2/25 Günther Schmidt
Hi everyone,
in my attempts to remove boilerplate and thus to do more "abstraction" I come across a number of interesting things and suggestions.
Especially blog posts from Dan Piponi and also Heinrich Apfelmus. I think what they both are saying is that you can construct / implement some sort of 2 layered monad which can then have more than one interpretation.
In the responses to one of my posts on DSLs Dan Piponi also points out that he considers monads also to be DSLs. It didn't click with me when he said it, but reading more of his blog posts made me remember that.
Now I know this is probably something obvious to most haskellers, but to me it's not.
Did I understand this correctly so far?
Günther
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