[Haskell] build an interpreter on top of GHCi?
Hi All, Does anybody has the experience to built an interpreter on top of GHCi? What I want is to defined a my own interpreter as a Haskell module and load it into GHCi. So this new interpreter will be running on top of GHCi which accepts syntax extension of Haskell. For example: Prelude> :l myInter Prelude> myInter> new language source Prelude> myInter> ... Prelude> myInter> exit Prelude> In another word, I want to have a read-eval-print-loop running on GHCi. Does anybody know how to do that? -W-M- @ @ | \_/ ___ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell
[Haskell] parser for "Typing Haskell in Haskell"
Hi All, Does anybody know what parser "Typing Haskell in Haskell" use? I am tring to use the code for some type checking. But I cannot find a parser in the distribution. -W-M- @ @ | \_/ ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
constrained datatype
Hi All, Anybody knows why the following code does not work? > class Foo n > data Erk n = Foo n => Erk test.hs:53: All of the type variables in the constraint `Foo n' are already in scope (at least one must be universally quantified here) When checking the existential context of constructor `Erk' In the data type declaration for `Erk' Failed, modules loaded: none. Is there any reason for this error? -W-M- @ @ | \_/ ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
import modules from a different directory
Hi All, Anybody has the experience of importing a module from a different directory?In my code, i need to import a module that is not in the current directory. How can I do that in GHC? -W-M- @ @ | \_/ ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: *safe* coerce, for regular and existential types
I admire the elegancy of your code which makes the changes to add new data types minimum. There is one question I want to ask: Does this technique extend to polymophic types? Let's say we have the following type: > data D a = C | D a Is it possible to index the type D a? Or there is some fundmental limitations which make it not achievable by Haskell type classes? -W-M- @ @ | \_/ On Thu, 31 Jul 2003 [EMAIL PROTECTED] wrote: > > This message describes functions safeCast and sAFECoerce implemented > in Haskell98 with common, pure extensions. The functions can be used > to 'escape' from or to existential quantification and to make > existentially-quantified datatypes far easier to deal with. Unlike > Dynamic, the present approach is pure, avoids unsafeCoerce and > unsafePerformIO, and permits arbitrary multiple user-defined typeheaps > (finite maps from types to integers and values). > > An earlier message [1] introduced finite type maps for > purely-functional conversion of monomorphic types to unique > integers. The solution specifically did not rely on Dynamic and > therefore is free from unsafePerformIO. This message shows that the > type maps can be used for a safe cast, in particular, for laundering > existential types. The code in this message does NOT use > unsafePerformIO or unsafeCoerce. To implement safe casts, we define a > function sAFECoerce -- which works just like its impure > counterpart. However the former is pure and safe. sAFECoerce is a > library function expressed in Haskell with common extension. The > safety of sAFECoerce is guaranteed by the typechecker itself. > > This whole message is self-contained, and can be loaded as it is in > GHCi, given the flags > -fglasgow-exts -fallow-undecidable-instances -fallow-overlapping-instances > > This message was inspired by Amr Sabry's problem on existentials. In > fact, it answers an unstated question in Amr Sabry's original message. > > > It has been observed on this list that existentially-quantified > datatypes are not easy to deal with [2]. For example, suppose we have > a value of a type > > > data EV = forall a. (TypeRep a TI)=> EV a > > (please disregard the second argument of TypeRep for a moment). > > The constructor EV wraps a value. Suppose we can guess that the > wrapped value is actually a boolean. Even if our guess is correct, we > *cannot* pass that value to any function of booleans: > > *> *Main> case (EV False) of (EV x) -> not x > *> > *> :1: > *> Inferred type is less polymorphic than expected > *> Quantified type variable `a' is unified with `Bool' > *> When checking an existential match that binds > *> x :: a > *> and whose type is EV -> Bool > *> In a case alternative: (EV x) -> not x > > A quantified type variable cannot be unified with any regular type -- > or with another quantified type variable. Values of existentially > quantified types cannot be passed to monomorphic functions, or to > constrained polymorphic functions (unless all their constrains have > been mentioned in the declaration of the existential). That limitation > guarantees safety -- on the other hand, it significantly limits the > convenience of existential datatypes [2]. > > To overcome the limitation, it _seemed_ that we had to sacrifice > purity. If we are positive that a particular existentially quantified > value has a specific type (e.g., Bool), we can use unsafeCoerce to > cast the value into the type Bool [3]. This approach is one of the > foundations of the Dynamic library. The other foundation is an ability > to represent a type as a unique run-time value, provided by the > methods of the class like TypeRep. Given an existentially quantified > value and a value of the desired type, Dynamic compares type > representations of the two values. If they are the same, we can > confidently use unsafeCoerce to cast the former into the type of the > latter. > > This works, yet leaves the feeling of dissatisfaction. For one thing, > we had to resort to an impure feature. More importantly, we placed our > trust in something like TypeRep and its members, that they give an > accurate and unique representation of types. But what if they lie to > us, due to a subtle bug in their implementation? What if they give the > same representation for two different types? unsafeCoerce will do its > dirty work nevertheless. Using the result would lead to grave > consequences, however. > > This message describes sAFECoerce and the corresponding safe cast. > Both functions convert the values of one type into the target type. > One or both of these types may be existentially quantified. When the > source and the target types are the same, both functions act as the > identity function. The safe cast checks that the type representations > of the source and the target types are the same. If they are, it > invokes sAFECoerce. Otherwise, we monadically fail. The function > sAFECoerce does the conversion without any type checkin
dynamics on polymorphic datatype
Hi All, I am trying to perform dynamic casting on polymorphic types. Let's say I have a data type like: > data Foo a = Foo a Is there any way to use dynamics to convert a value of type Foo a to a type reprentation? I try to use the toDyn in the dynamic libray, it complains for ambigours a. Is there a solution to it? Thank you very much. -W-M- @ @ | \_/ ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
polymorphic type in state of state monad
Hi All, Any one of your have the experience of defining a state of a state monad as a polymorphic type? I want to have: > type State = Term a => [a] > data M a = M (State -> IO(State,a)) GHC yields a error message "Illegal polymorphic type". How to resolve this? Thank you very much. -W-M- @ @ | \_/ ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
overload lift function
Hi All, I have a data type defined as > data D a = D1 Int | D2 a Follow this definition, I have D2 Int,D2 Bool and D2 [D2 Int] as instances of type D. I want to write an overloaded function lift which lifts an atom type to D. It is easily done for D2 Int and D2 Bool using type classes.I can have > class Liftable a where > lift :: a -> L a > > instance Liftable Int where > lift x = D2 x However, I have problem with D2 [D2 Int].I want to lift [Int] to D2 [D2 Int]. Is there any solution to it? Thank you very much. -W-M- @ @ | \_/ ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: int to float problem
Try intToFloat :: Int -> Float intToFloat n = fromInteger (toInteger n) -W-M- @ @ | \_/ On Fri, 28 Feb 2003, Mike T. Machenry wrote: > Hello, > > I am having a problem. I recently desided I wanted a bunch function to return > float instead of Int. I changed their type and wrote a new function that > returned a float. I figured it'd be okay if all the others still returned > Int since it's trivial to convert Int to Float. Sadly Haskell won't let me do > this. What should I do? I attempted to cast all of the values in the functions > that returned Int to Float, but I found no way to do this. I found fromInteger > but it didn't seem to work on the return value of the cardinality function for > instance. I guess cardinality much return Int. This is one of my functions. > > smallestSet :: GameState -> Float > smallestSet s = (-1 * cardinality (fLocations s)) > > This function is an error because it infer's an Int. > > Thanks, > -mike > ___ > Haskell mailing list > [EMAIL PROTECTED] > http://www.haskell.org/mailman/listinfo/haskell > ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
escape from existential quantification
I understand that existentially bound types cannot escape. For example, say we have data Foo = forall a. Foo Int a Then we cannot define a function extract (Foo i a) = a However,this limitation makes it extremly difficult to program with local quantifications.Is there any way to by pass this? -W-M- @ @ | \_/ ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
