Re: Pattern Matching

On Nov 12, 2009, at 21:15 , Casey Hawthorne wrote:
> Why in a pattern match like
>
> score (1 3) = 7
>
> can I not have
>
> sizeMax = 3
>
> score (1 sizeMax) = 7

Because it's a pattern, and when you introduce a symbol you are  
inviting the pattern match to bind what it matched to that name for  
use within the function.  (Ordinary arguments are a trivial case of  
this.)

Or, by example:

 > score (1 sizeMax) = (expression using sizeMax)

The normal way to do what you want is guards:

 > score (1 x) | x == sizeMax = 7 -- you can pronounce the "|" as  
"such that"

--

-- 
brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery <at> kf8nh.com
system administrator [openafs,heimdal,too many hats] allbery <at> ece.cmu.edu
electrical and computer engineering, carnegie mellon university    KF8NH

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Re: Pattern Matching

Casey,

> Why in a pattern match like
> 
> score (1 3) = 7

You probably mean
> score 1 3 = 7

which applies the function 'score' to two arguments.  With the parentheses,
it looks like an application of '1' to the argument '3'.  But to answer your
actual question...

> can I not have
> 
> sizeMax = 3
> 
> score (1 sizeMax) = 7

When a variable name (such as 'sizeMax') appears in a pattern, it gets bound
there.  This is useful so you can refer to the variable on the right hand
side, as in:

successor x = x + 1

How would the compiler know whether to bind the variable (what actually
happens), or match against the value represented by some earlier binding
(what you're asking for)?

What if the type of that second argument doesn't have a defined equality
operation?

There might be some reasonable way to do it, but I suspect it would be
fragile and error-prone, at best.  So it's always a new binding.  It's easy
enough to work around:

score 1 x | x == sizeMax = 7

Regards,
John

Re: Pattern Matching

*

Casey Hawthorne wrote:
> Why in a pattern match like
>
> score (1 3) = 7
>
> can I not have
>
> sizeMax = 3
>
> score (1 sizeMax) = 7
>   

If I had a dollar for every time I've written something like

  case msg of
    eVENT_QUIT -> ...
    eVENT_POST -> ...
    eVENT_RESIZE -> ...

and spent an hour trying to figure out why the messages aren't being 
processed right... 

Re: Pattern Matching

Thank you to all who replied, very instructive.
--
Regards,
Casey