# Incoherence

**John Hughes
**
rjmh@cs.chalmers.se

*Fri, 26 Oct 2001 09:02:30 +0200 (MET DST)*

John Hughes wrote:
| `x := []' wouldn't be problematic, just monomorphic.
| That is, x must be used consistently as a list of a
| particular type.
Just to check if I understand you correctly. In your
proposal, does the following thing share `x'?
let x = fac 100 in x + x
(My understanding is that, in order to do this in your
proposal, we have to say:
let x := fac 100 in x + x
Correct?)
Correct.
Does this also mean that it becomes impossible to share
polymorphic values? I.e. the following contrived example
will not behave as usual:
wrong :: Either String a
wrong = Left (show (fac 100)) -- not shared?
example1 :: Either String Int
example1 :=
do wrong
return 1
example2 :: Either String Char
example2 :=
do wrong
return 'a'
(With the usual Either monad instance.)
Also true.
Of course, in this case, there is a perfect remedy, just
lifting out `show (fac 100', which is a monomorphic
expression.
However, this is not always possible, we might have a
datatype like this:
data PolyMonoList a
= Poly Int (Foo a)
| Mono a (Foo a)
| Nil
We can construct rather large polymorphic values, which have
to be converted in linear time to the same value of a
different type.
Yup.
I guess we already have the problem of not being able to
share monomorphic values polymorphically. In fact, the
Either monad instance is a good example:
instance Monad (Either e) where
return = Right
Left x >>= k = Left x
Right a >>= k = k a
In the definition of `>>=', it is impossible to share `Left
x'.
/Koen.
All true, but I doubt whether any of this is a big deal. MLers already live
with their "value restriction", which has similar penalties, so there is a
precedent.
John