[Haskell-cafe] Bool as type class to serve EDSLs.
simonpj at microsoft.com
Thu May 28 03:14:28 EDT 2009
You are absolutely right about the tantalising opportunity. I know that Lennart has thought quite a bit about this very point when designing his Paradise system. Likewise Conal for Pan.
One difficulty is, I think, that it's easy to get ambiguity. Eg
ifthenelse (a > b) e1 e2
The (a>b) produces a boolean-thing, and ifthenelse consumes it; but which type of boolean? The Expr type? Real Bools? Or what?
If there was a nice design, then GHC's existing -fno-implicit-prelude flag could be extended (again) to desugar if-then-else to the new thing. But the design is unclear, to me anyway.
| -----Original Message-----
| From: haskell-cafe-bounces at haskell.org [mailto:haskell-cafe-bounces at haskell.org] On
| Behalf Of Sebastiaan Visser
| Sent: 27 May 2009 13:32
| To: Haskell Cafe
| Subject: [Haskell-cafe] Bool as type class to serve EDSLs.
| While playing with embedded domain specific languages in Haskell I
| discovered the Num type class is a really neat tool. Take this simple
| example embedded language that can embed primitives from the output
| language and can do function application.
| >data Expr :: * -> * where
| > Prim :: String -> Expr a
| > App :: Expr (a -> b) -> Expr a -> Expr b
| Take these two dummy types to represent things in the output language.
| >data MyNum
| >data MyBool
| Now it is very easy to create an Num instance for this language:
| >primPlus :: Expr (MyNum -> MyNum -> MyNum)
| >primPlus = Prim "prim+"
| >instance Num (Epxr MyNum) where
| > a + b = primPlus `App` a `App` b
| > fromInteger = Prim . show
| > ...
| Which allows you to create very beautiful expression for your language
| embedded inside Haskell. The Haskell expression `10 * 5 + 2' produces
| a nice and well typed expression in your embedded domain.
| But unfortunately, not everyone in the Prelude is as tolerant as the
| Num instance. Take the Eq and the Ord type classes for example, they
| require you to deliver real Haskell `Bool's. This makes it impossible
| make your DSL an instance of these two, because there are no `Bool's
| only `Expr Bool's.
| Which brings me to the point that, for the sake of embedding other
| languages, Haskell's Prelude (or an alternative) can greatly benefit
| from (at least) a Boolean type class like this:
| class Boolean a where
| ifthenelse :: a -> b -> b -> b -- Not sure about this
| And one instance:
| >instance Boolean (Expr MyBool) where
| > ifthenelse c a b = Prim "if-then-else" `App` c `App` a `App` b
| Now we can change (for example) the Eq type class to this:
| >class Eq a where
| > (==) :: Boolean b => a -> a -> b
| > (/=) :: Boolean b => a -> a -> b
| For which we can give an implementation for our domain:
| >primEq :: Epxr (a -> a -> MyBool)
| >primEq = Prim "=="
| >instance Eq (Expr a) where
| > a == b = primEq `App` a `App` b
| And now we get all functionality from the Prelude that is based on Eq
| (like not, &&, ||, etc) for free in our domain specific language! Off
| course there are many, many more examples of things from the standard
| libraries that can be generalised in order to serve reuse in EDSLs.
| Anyone already working on such a generalized Prelude? I can imagine
| much more domains can benefit from this than my example above. Any
| interesting thoughts or pointers related to this subject?
| Sebastiaan Visser
| Haskell-Cafe mailing list
| Haskell-Cafe at haskell.org
More information about the Haskell-Cafe