On Tue, 19 Jun 2001, Wolfgang Lux wrote:

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| Another simple alternative is to weaken the the equation by | replacing the variable r by the empty string: [Alternative B:] | | ... | should be the case that | | fst (head (readsPrec d (showsPrec d x ""))) == x

Yes, I like this alternative too. I'll implement it.

Why not require

readsPrec d (showPrec d x "") == [(x,"")]

in this case? At least it should be

head (readsPrec d x "") == (x,"") [Typo - you mean:] head (readsPrec d (showsPrec d x "")) == (x,"")

Both formulations look OK to me, but neither is an essential change and there are many other small variations possible with slight semantical differences. One example is (x,"") `elem` readsPrec d (showsPrec d x "") And already the current (proposed) formulation is most likely impossible to satisfy if the element x contains bottoms somewhere inside. (Example: try to show and read the string (undefined : "hello").) I think we'll have to accept that Show and Read are to be used mainly for presentation, and not when real uniquely invertible behaviour is desired. I think a separate Serializable class designed for correct and portable (and efficient?) data transmission is necessary anyway. In that design process the laws would have to be thought through much more carefully. (There are suggestions for a library already - see below.) /Patrik A quick search: http://www.cs.york.ac.uk/fp/nhc98/libs/Binary.html The Bits Between The Lambdas: Binary Data in a Lazy Functional Language (1998) Malcolm Wallace Colin Runciman http://citeseer.nj.nec.com/260182.html Heap Compression and Binary I/O in Haskell Malcolm Wallace Colin Runciman http://citeseer.nj.nec.com/68962.html