"interact" behaves oddly if used interactively

[Koen Claessen]

Simon Marlow wrote:

 | For example, eager evaluation would be a completely
 | valid implementation strategy for Haskell if it were
 | not for lazy I/O.

I do not understand this remark.

As far as I know, in any valid implementation of Haskell,
the following expression:

  const 3 undefined

should always produce 3; any valid evaluation strategy for
Haskell should respect not trying to evaluate something like
undefined if it is not needed for the computation.

I see no difference between that and that any valid Haskell
implementation should avoid trying to read a character from
the input if it is not needed.

/Koen