[Haskell-cafe] Haskell Style - Pattern Matching with case vs. function declarations

Luke Palmer 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:
> [code]
> foldl' _    zero []     = zero
> foldl' step zero (x:xs) =
>      let new = step zero x
>      in  new `seq` foldl' step new xs
> [/code]

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
pass up.

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