[Haskell-cafe] IO is a bad example for Monads [was: do]
dpiponi at gmail.com
Tue Dec 4 00:11:13 EST 2007
On Dec 3, 2007 6:36 PM, Ben Franksen <ben.franksen at online.de> wrote:
> then the special features of IO
> will remain associated with monads in general, leading to a whole jumble of
> completely wrong ideas about them.
As I only learnt about monads a couple of years ago, the process is
still fresh in my mind. I wasted quite a bit of time labouring under
the impression that monads were primarily about sequencing. But that
wasn't because I incorrectly generalised from IO. It was because
countless people out there explicitly said they were about sequencing.
I suspect that if courses started with the List monad there'd be
countless blogs telling people that monads are a way to eliminate
loops from your code like the way list comprehensions are used in
> This is yet another problem with IO as the standard example for monads: its
> effect base is huge and poorly structured.
You don't teach *all* of IO to students in one go!
> This again makes it difficult to
> see exactly which intuitions about IO can be generalized to arbitrary
> monads and which not.
That's true of any monad. IO is unique.  is unique. Cont is unique.
All of them can lead you down the garden path. You need to see
multiple monads, and it helps if you can sneak an example under a
student's nose so they can already reason about monads before they
even know what a monad is.
> What is pointless about failure and how to handle it?
It's pointless when you're still trying to make your first tweaks to
"Hello, World!" work.
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