[Haskell-cafe] Haskell Style - Pattern Matching with case vs.
lrpalmer at gmail.com
Wed Mar 18 23:12:36 EDT 2009
On Wed, Mar 18, 2009 at 11:49 AM, Tom.Amundsen <tomamundsen at gmail.com>wrote:
> Hi All,
> I am new to Haskell. I just started reading "Real World Haskell" a few days
> ago, so I apologize for being such a noob.
> But, I am curious why I see a lot of code where people do pattern matching
> via multiple function declarations instead of using the case ... of ...
> construct? For example:
> foldl' _ zero  = zero
> foldl' step zero (x:xs) =
> let new = step zero x
> in new `seq` foldl' step new xs
Well, in this particular case, note that you have two equations written.
These equations are true of foldl', and in fact foldl' is the *least defined
* function satisfying these equations (see
http://en.wikibooks.org/wiki/Haskell/Denotational_semantics for the
underlying theory of "definedness"). It a very pretty idea.
However, this happy view of equations breaks down once you start using the
"fall through" semantics, as in:
foo (x:y:xs) = x + y
foo xs = 0
In which the second equation does not always hold (foo [1,2,3] = 0 is
false). Because of this, I am beginning to prefer not writing functions
using "fall through", although occasionally it is too damned convenient to
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