Missing class functions

[Graham Klyne]

I came across a surprising feature of Haskell's class system.

If I declare an instance of a class, but do not define one of the functions 
of that class, the compiler does not notice.  Even if I try to reference 
the undefined function, the compiler doesn't notice.  It's only when I try 
to evaluate the missing function, at run-time,  that I get an error message.

I now see this is correct behaviour:
[[
If no binding is given for some class method then the corresponding default 
class method in the class declaration is used (if present); if such a 
default does not exist then the class method of this instance is bound to 
undefined and no compile-time error results.
]]
-- [Report, section 4.3.2]

Is there a rationale for allowing class methods to be undefined for an 
instance?

#g
--

[[
-- Missing class functions not detected?
-- SpikeMissingClassFunctions.hs

class (Show (m a), Show a) => C m a where
     a1 :: [a] -> m a
     a2 :: m a -> [a]
     a3 :: m a -> Bool

data MyC a = MyC [a]

instance (Show a) => C MyC a where
     a1 s         = MyC s
     a2 (MyC s)   = s

instance (Show a) => Show (MyC a) where
     show (MyC s) = "MyC "++(show s)

test1 = a1 [1,2,3] :: MyC Int     -- "MyC [1,2,3]"
test2 = a2 test1                  -- "[1,2,3]"
test3 = a3 test1                  -- "Program error: Undefined member: a3"
]]

Hugs log:
[[
Reading file "D:\Cvs\DEV\HaskellRDF\Spike\SpikeMissingClassFunctions.hs":

Hugs session for:
C:\DEV\Hugs98\libraries\Hugs\Prelude.hs
C:\DEV\Hugs98\libraries\Prelude.hs
D:\Cvs\DEV\HaskellRDF\Spike\SpikeMissingClassFunctions.hs
Main> test1
MyC [1,2,3]
Main> test2
[1,2,3]
Main> test3

Program error: Undefined member: a3
]]


-------------------
Graham Klyne
<GK@NineByNine.org>
PGP: 0FAA 69FF C083 000B A2E9  A131 01B9 1C7A DBCA CB5E