Pattern guards

Yitzchak Gale gale at
Sat Sep 30 21:23:15 EDT 2006

Conor McBride wrote:

> Claus Reinke wrote:

>> ...the results of the translation are rather
>> more awkward but -and this is the important
>> point- pattern guards do not add new
>> functionality.

> Well, neither do Boolean guards nor even basic
> pattern matching...  one simply should not need
> to clutter a program with do, return, mplus and
> fromJust (ugh!), spelling out the semantics of
> pattern matching in minute detail. For at least
> 36 years, we've been able to hide all that junk
> behind a highly readable equational notation.
> This is one monad we don't need to see.

Some complex things are happening: selections and
bindings are happening at the same time.  The
monad spells it out clearly and concisely, without
adding very much weight at all.  Function
definitions appear visually almost the same,
with or without the pattern guards.

There has to be a really, really compelling reason
to add new syntax to a language. Every bit of new
syntax makes a language harder to learn, and less
usable for the general user.

> without pattern guards, we're forced to the
> right if we want to examine the result of an
> intermediate computation. This means we have to
> do any subsequent analysis using the syntax of
> expressions which is much clunkier than that of
> left-hand sides.

Nice point.


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