ghci: catching up with hugs?-)

Claus Reinke claus.reinke at
Thu Jul 29 12:48:26 EDT 2004

Having mostly been converted from hugs to ghci (due to working
with too large programs or libraries more readily supported there),
I've come to find it quite indispensible for Haskell development.

However, there are a few corners where I still miss Hugs functionality
that doesn't seem to be available in ghci, and I wonder whether there
are any plans to remedy this (or whether there are any fundamental

- especially when working with gui libs, I often find myself wanting to 
    know which instances some type belongs to (as that determines the
    attributes/properties/etc one may use with that type).

    in hugs, a simple ":info <type>" gives me the answer:

        Prelude> :info Bool
        -- type constructor
        data Bool
        -- constructors:
        False :: Bool
        True :: Bool
        -- instances:
        instance Eq Bool
        instance Ord Bool
        instance Ix Bool
        instance Enum Bool
        instance Read Bool
        instance Show Bool
        instance Bounded Bool

    in fact, I can even ask by class, using ":info <class>":

        Prelude> :i Enum
        -- type class
        class Enum a where
          succ :: a -> a
          pred :: a -> a
          toEnum :: Int -> a
          fromEnum :: a -> Int
          enumFrom :: a -> [a]
          enumFromThen :: a -> a -> [a]
          enumFromTo :: a -> a -> [a]
          enumFromThenTo :: a -> a -> a -> [a]
        -- instances:
        instance Enum ()
        instance Enum Char
        instance Enum Int
        instance Enum Integer
        instance Enum Float
        instance Enum Double
        instance Integral a => Enum (Ratio a)
        instance Enum Bool
        instance Enum Ordering

    in ghci, such useful info seems absent, and I find myself hunting
    the haddocs. shouldn't ghci be able to provide this info as well?

- since ghc now keeps better source location info, how about ":find <name>"?

    in hugs, that calls a configurable external command (usually an editor)
    with the filename and linenumber of <name>'s definition. this is very 
    useful, even more so for Haskell IDEs that communicate with hugs to
    implement "jump to definition" instead of relying on outdated or non-
    existent tag files.

    in ghci, that could simply pass filename and linenumber to a configurable
    Haskell function (:: String -> Int -> IO ()) - well, if we have the source code..

    so we need an indication of whether ghci knows where to find the source, 
    and if not, we need the package and full hierarchical module and item name, 
    so that the user-supplied "find" function could eg. try to open the 
    corresponding haddoc entry if the source is not available, before giving 
    up. simplified example:

    myfind :: Maybe (FilePath,Int) -> (String,String) -> IO ExitCode
    myfind (Just (filepath,line)) _ = 
        system $ editor++" +"++show line++" "++filepath
    myfind Nothing (package,mp) = 
        system $ browser++" "++haddocBase++sep++package++sep++mp++".html"

    :set find myfind
    :find MVar
    -- goes off to open "<somewhere>\base\Control.Concurrent.MVar.html"

    [ghci already offers source location information as part of ":info", *if* it
     has seen the source code; I'm just asking for a more convenient interface,
     as well as infomation in the common case of no source code]

- the ":set <something>" command in ghci "feeds" the ghc command line, but how
    can I figure out the current settings (especially paths and packages)? 

    in hugs ":set" would list all settings; in ghci that doesn't seem to happen.

Especially the first two (instance info and configurable source/doc link 
would help me with frequently recurring work patterns during Haskell 
code development.


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