seq as a class method
Wolfgang Jeltsch
wolfgang at jeltsch.net
Thu Mar 23 09:52:54 EST 2006
Hello,
it seems that there is not yet a ticket about putting seq into a type class
(again).
In my opinion, having seq available for every type is a serious flaw. One
problem is that the law f = \x -> f x doesn't hold anymore since the equation
is false for f = _|_. There was also a discussion on one of the mailing
lists some time ago which revealed that the Monad instance for IO doesn't
satisfy the monad laws because of the availability of seq for IO, I think.
In addition, the automatic definition of seq for every type can make
implementation details visible which were thought of as completely hidden.
For example, it might make a difference whether one uses data or newtype for
a one-alternative-one-field datatype, even if the data constructor is hidden.
I therefore propose to declare a class like this:
class Seq a where
seq :: a -> b -> b
(Perhaps the class name should be chosen better.) Generation of standard
instances should be supported via deriving clauses or Template Haskell. For
an algebraic datatype declared via
data T a1 ... an = C1 t11 ... t1m1 | ... | Ck tk1 ... tkmk
the implementation of seq should be something like this:
seq (C1 _ ... _) = id
seq _ = id
What do others think?
Best wishes,
Wolfgang
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