Hey everyone; recently I've been toying around with various methods of writing a shell and reading the academic literature on such things. The best prior art on the subject seems to be the ESTHER shell (see <http://citeseer.ist.psu.edu/689593.html>, <http://citeseer.ist.psu.edu/744494.html>, <ftp://ftp.cs.kun.nl/pub/Clean/papers/2003/vWeA2003-Esther.pdf>).
Now, ESTHER is a really cool looking shell, but it has two main problems for me: 1) Source doesn't seem to be available anywhere online 2) It's written in Clean and not Haskell No problem. All the hard stuff is done, and there's like a good 50 pages of documentation, so how hard could it be? Clean is practically Haskell anyway. But immediately I ran into a road-block: "The shell is built on top of Clean's hybrid static/dynamic type system and its dynamic I/O run-time support. It allows programmers to save any Clean expression, i.e a graph that can contain data, references to functions, and closures to disk. Clean expressions can be written to disk as a _dynamic_, which contains a representation of their (polymorphic) static type, while preserving sharing. Clean programs can load dynamics from disk and use run-time type pattern matching to reintegrate it into the statically-typed program." The Data.Dynamic library seems to do everything as far as dynamic types and run-time pattern matching goes, but I haven't figured out how one could write Haskell expressions to disk, like Clean's system <http://www.st.cs.ru.nl/papers/2002/verm2002-LazyDynamicIO.ps.gz> apparently allows. Does anyone know if there are any neat or tricky ways this could be done? Projects, extensions, whatever? On #haskell, quicksilver did tell me of one neat way to serialize various stuff through Data.Binary by using ADTs along the lines of the following simple example: -- module Main (main) where import Data.Binary data Math = Add | Subtract | Multiply deriving Show eval :: (Num a) => Math -> a -> a -> a eval f = case f of Add -> (+) Subtract -> (-) Multiply -> (*) instance Binary Math where put Add = putWord8 0 put Subtract = putWord8 1 put Multiply = putWord8 2 get = do tag_ <- getWord8 case tag_ of 0 -> return Add 1 -> return Subtract 2 -> return Multiply main = do encodeFile "tmp.s" [Add, Subtract, Multiply] a <- decodeFile "tmp.s" putStr $ show (a :: [Math]) -- Since from my Lisp days I know that code is data, it strikes me that one could probably somehow smuggle Haskell expressions via this route although I am not sure this is a good way to go or even how one would do it (to turn, say, a list of the chosen ADT back into real functions, you need the 'eval' function, but apparently eval can only produce functions of the same type - so you'd need to either create as many adts and instances as there are varieties of type signatures in Haskell '98 and the libraries, I guess, or somehow encode in a lambda calculus). Is that a route worth pursuing? -- gwern
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