Re: [Haskell-cafe] Martin Odersky on "What's wrong with Monads"

Any idea whether Martin Odersky has read this discussion? Thanks and regards, -Damodar Kulkarni On Fri, Jun 29, 2012 at 12:23 AM, Dominique Devriese < dominique.devriese@cs.kuleuven.be> wrote:

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MightyByte wrote:

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Of course every line of your program that uses a Foo will change if you switch to IO Foo instead.

But we often have to also change lines that don't use Foo at all. For example, here is the type of binary trees of integers:

data Tree = Leaf Integer | Branch (Tree Integer) (Tree Integer)

A function to add up all integers in a tree:

amount:: Tree -> Integer amount (Leaf x) = x amount (Branch t1 t2) = amountt1 + amountt2

All fine so far. Now, consider the following additional requirement: "If

2012/6/27 Tillmann Rendel : the

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command-line flag --multiply is set, the function amount computes the product instead of the sum."

In a language with implicit side effects, it is easy to implement this. We just change the third line of the amount function to check whether to call (+) or (*). In particular, we would not touch the other two lines.

How would you implement this requirement in Haskell without changing the line "amount (Leaf x) = x"?

I may be missing the point here, but having worked on large code bases with a wide variety contributors before, I find it very advantageous that programmers are prevented from writing an amount function whose behaviour depends on command line arguments without at least an indication in the type. The fact that the function can not perform stuff like that is precisely the guarantee that the Haskell type gives me...

Dominique

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