[Haskell-beginners] 'Simple' function

[Rein Henrichs]

On Wed, Jun 10, 2015 at 11:15 AM Marcin Mrotek <marcin.jan.mrotek at gmail.com>
wrote:

> That's why, this is not a pure function because every time you call it, it
> may return a different string.
>

This is a common source of confusion. A value of type IO a for some a is
not an impure function because it is not a function. Its *evaluation* is
completely pure and referentially transparent: every time you evaluate
`getLine`, you get the same IO String value. The only observable difference
is under execution, but *we don't expect execution to be pure*: we only
expect evaluation to be pure.
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