Archive for 30th March 2009

Notions of purity in Haskell

Lately I’ve been learning that some programming principles I treasure are not widely shared among my Haskell comrades. Or at least not widely among those I’ve been hearing from. I was feeling bummed, so I decided to write this post, in order to help me process the news and to see who resonates with what I’m looking for.

One of the principles I’m talking about is that the value of a closed expression (one not containing free variables) depends solely on the expression itself — not influenced by the dynamic conditions under which it is executed. I relate to this principle as the soul of functional programming and of referential transparency in particular.


  • 2009-10-26: Minor typo fix

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