Confused about default

[Simon Peyton-Jones]

Well, Foo *is* an instance of Num, so a correct Haskell impl
should pick that instance always.  Since you don't define the
fromInteger method in the instance decl, you get a runtime error.

I don't know what you expect the [Foo] and Foo -> Int defaults
to do.

GHCi fails to put the correct default decl in place for command-line
expressions.  This is a known bug which we have not got around
to fixing yet

Simon

| -----Original Message-----
| From: Ian Lynagh [mailto:igloo@earth.li]=20
| Sent: 14 December 2001 21:29
| To: haskell@haskell.org
| Subject: Confused about default
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| Hi all
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| I am rather confused about default.
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| In section 4.3.4 the report says
|     "each ti must be a monotype for which Num ti holds"
| but according to grep this is the only place "monotype"=20
| appears in the report.
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| If I have the module
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|     module TT (Foo(..)) where
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|     default (Foo, [Foo], Foo -> Int)
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|     data Foo =3D Foo deriving (Eq, Show)
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|     instance Num Foo where {}
|     instance (Eq a, Show a) =3D> Num [a] where {}
|     instance Eq (a -> b) where {}
|     instance Show (a -> b) where {}
|     instance Num (a -> b) where {}
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| then hugs accepts it and
|     TT> 5
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|     Program error: Undefined member: fromInteger
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| ghc accepts it and
|     TT> 5
|     5
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| nhc tells me
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|     Error when renaming::
|     Illegal type in default at 4:17
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| If I remove all the list and function stuff then hi says
|     TT> 5
|     5
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| If I have a
|     main =3D putStrLn $ show 5
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| then nhc gives me
|     TT> main
|     No default definition for class method fromInteger
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| and ghci gives me
|     Main> main
|     *** Exception: TT.lhs:8: No instance nor default method for class
|     operation PrelNum.fromInteger
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| It seems to me that hugs is in the right with it's handling=20
| of 5, but I am not sure who is wrong with default ([Foo]) or=20
| (Foo -> Int).
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| Finally, the context free grammar doesn't currently enforce=20
| the restriction that only one default declaration be given.=20
| Fixing it would make rather a mess, though.
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| Ian
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