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Re: [Haskell-cafe] Why?

Subject: Re: [Haskell-cafe] Why?
From: wren ng thornton
Date: Thu, 10 Dec 2009 09:22:50 -0500
John D. Earle wrote:
This is a matter that I genuinely at the present time do not grasp and I am hoping that 
some of you who are more familiar with the Haskell language may be able to help enlighten 
me. I feel the question to be an important one. What material benefit does Haskell derive 
from being a "pure" functional language as opposed to an impure one? Please 
provide examples as I require instruction.

The following is what I believe to be true at the present time. It seems to be 
that the decision was made because it was a matter of taste under the belief 
that computer scientists can and often are superstitious and their 
superstitions can and often do materially interfere with progress. What I am 
saying is that at the present time perhaps due to my ignorance I am unfamiliar 
with how this benefits the language in a material sense. It appears to be a 
philosophical matter, a matter of identity, what Haskell stands for.

The sort of decision that Apple computer and Microsoft made not to go down the 
POSIX road seems relevant. Historically, Apple did not embrace POSIX. Windows 
continues to stand for Windows, that is the graphical user interface.

Laziness, referential transparency, equational reasoning,... They're excellent things, but how about a pragmatic example?

I was working recently on a metaprogramming framework to automate the generation of a bunch of shell scripts for wiring programs together. To ensure bug-free scripts we want to maintain a few invariants. One invariant is that the names of files to be generated should not be accessible prior to the file actually being generated (to avoid the file equivalent of a null-pointer dereference). Another invariant is that if someone wants to run a particular generated script then we should ensure that all prerequisite scripts are run first (a la any other build system).

After some familiarity with Haskell it's easy to see that what we want here is a monad. This particular monad keeps track for each value (i.e., script) the prerequisite values necessary for it to be valid (i.e., tracking all other values used in constructing this one). The monad laws ensure that no values can escape--- but only with purity. If we could, for example, make global assignments without altering the type signatures then it would be possible to smuggle a file name out of the monadic scope, and thus to violate our invariant about names only being accessible when they're valid. These sorts of issues were painfully apparent because I was writing this framework in an impure language for non-Haskellites to use.

There are many other examples along these lines as well. In general, purity means we can actually use mathematical notions in a rigorous way to ensure the program behaves appropriately and are not susceptible to various logic bugs. Referential transparency and equational reasoning are just two special-case examples of this general benefit. Having monads actually be monads is another less recognized one.

Live well,
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