| -----Original Message----- | From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of John | Hughes | Sent: 10 January 2005 09:16 | To: haskell-cafe@haskell.org | Subject: [Haskell-cafe] Re: Some random newbie questions | | I seriously considered switching frlom Hugs to GHC for my introductory | programming class this year, but in the end stayed with Hugs because of | a single feature. | | I'm teaching beginning programmers, and for them at least, there is an | overwhelming volume of names to learn -- "what's that function?" is a | question they ask themselves often, as is "what's that type?". I teach | them that, whenever they see a name they don't recognise, they can find | out more about it using the :i command. This is a simple trick to learn, | that helps them understand points they've missed and catches | misapprehensions. | | My students also see type classes very early. I'll bet yours will too. | Even if one is very careful to restrict the examples in lectures so as | to avoid them (which is a bind), as soon as students try out Hugs for | themselves, they will make mistakes that generate error messages | referring to type classes. No problem: the question "what's that class?" | can ALSO be answered by :i. | | Now, at the beginning students have only a very rudimentary | understanding of classes. A class is a collection of types to them, | nothing more. In particular, the class definition itself is of little | use to them, since it often contains a very subtly chosen collection of | methods (just type :i Show, for example, which students do very early). | What IS useful, right from the beginning, is the list of instances. What | are Num types? Oh, integers and reals. What are Show types? Oh, pretty | much everything. Particularly when debugging "missing instance" errors, | this is just the information you need. | | Unfortunately, while Hugs prints the list of instances of a class in | response to :i, GHCi does not. It only prints the class definition -- | which, for my students, contains no useful information. For that reason | alone, I stuck with Hugs last year. | How about changing the behaviour of :i, Simon, so I can use GHCi | throughout next year? Done, some while ago. You want GHC 6.4! See below Simon Prelude> :i Show class Show a where showsPrec :: Int -> a -> String -> String show :: a -> String showList :: [a] -> String -> String -- Imported from `GHC.Show' instance Show IOMode -- Imported from `GHC.IOBase' instance Show IOException -- Imported from `GHC.IOBase' instance Show IOErrorType -- Imported from `GHC.IOBase' instance Show HandleType -- Imported from `GHC.IOBase' instance Show Handle -- Imported from `GHC.IOBase' instance Show ExitCode -- Imported from `GHC.IOBase' instance Show Exception -- Imported from `GHC.IOBase' instance Show BufferMode -- Imported from `GHC.IOBase' instance Show AsyncException -- Imported from `GHC.IOBase' instance Show ArrayException -- Imported from `GHC.IOBase' instance Show ArithException -- Imported from `GHC.IOBase' instance Show SeekMode -- Imported from `GHC.Handle' instance Show HandlePosn -- Imported from `GHC.Handle' instance Show CWchar -- Imported from `Foreign.C.Types' instance Show CUShort -- Imported from `Foreign.C.Types' instance Show CULong -- Imported from `Foreign.C.Types' instance Show CULLong -- Imported from `Foreign.C.Types' instance Show CUInt -- Imported from `Foreign.C.Types' instance Show CUChar -- Imported from `Foreign.C.Types' instance Show CTime -- Imported from `Foreign.C.Types' instance Show CSize -- Imported from `Foreign.C.Types' instance Show CSigAtomic -- Imported from `Foreign.C.Types' instance Show CShort -- Imported from `Foreign.C.Types' instance Show CSChar -- Imported from `Foreign.C.Types' instance Show CPtrdiff -- Imported from `Foreign.C.Types' instance Show CLong -- Imported from `Foreign.C.Types' instance Show CLLong -- Imported from `Foreign.C.Types' instance Show CLDouble -- Imported from `Foreign.C.Types' instance Show CInt -- Imported from `Foreign.C.Types' instance Show CFloat -- Imported from `Foreign.C.Types' instance Show CDouble -- Imported from `Foreign.C.Types' instance Show CClock -- Imported from `Foreign.C.Types' instance Show CChar -- Imported from `Foreign.C.Types' instance Show (GHC.Ptr.Ptr a) -- Imported from `Foreign.Ptr' instance Show (GHC.Ptr.FunPtr a) -- Imported from `Foreign.Ptr' instance Show Float -- Imported from `GHC.Float' instance Show Double -- Imported from `GHC.Float' instance Show Int8 -- Imported from `GHC.Int' instance Show Int64 -- Imported from `GHC.Int' instance Show Int32 -- Imported from `GHC.Int' instance Show Int16 -- Imported from `GHC.Int' instance Show Word8 -- Imported from `GHC.Word' instance Show Word64 -- Imported from `GHC.Word' instance Show Word32 -- Imported from `GHC.Word' instance Show Word16 -- Imported from `GHC.Word' instance Show Word -- Imported from `GHC.Word' instance Show [a] -- Imported from `GHC.Show' instance Show (a, b, c, d, e) -- Imported from `GHC.Show' instance Show (a, b, c, d) -- Imported from `GHC.Show' instance Show (a, b, c) -- Imported from `GHC.Show' instance Show (a, b) -- Imported from `GHC.Show' instance Show () -- Imported from `GHC.Show' instance Show Ordering -- Imported from `GHC.Show' instance Show (Maybe a) -- Imported from `GHC.Show' instance Show Int -- Imported from `GHC.Show' instance Show (Either a b) -- Imported from `GHC.Show' instance Show Char -- Imported from `GHC.Show' instance Show Bool -- Imported from `GHC.Show' Prelude>