Proposal: pattern synonyms

Conor McBride ctm at Cs.Nott.AC.UK
Thu Feb 16 06:10:59 EST 2006

Malcolm Wallace wrote:

>Conor McBride <ctm at> writes:
>>  P x y z = C x (y, [z])
>Isn't this idea very similar to views, and pattern-guards?

Clearly related, but rather cheaper. I don't want to make remarks, 
positive or negative about either of those proposals at this juncture. I 
see pattern synonyms, like type synonyms, as a convenient abbreviation 
mechanism in a part of the language where verbosity is currently compulsory.

>For instance, you could rewrite your example thus:
>data EffectView x = Bang
>                  | Bing x
>                  | Dull x
>view :: MyEffect x -> EffectView x
>view (Comp Nothing)                         = Bang
>view (Comp (Just (Prod (Any True), Id x)))  = Bing x
>view (Comp (Just (Prod (Any False), Id x))) = Dull x
>my_function e
>    | Bang   <- view effect  = ...
>    | Bing x <- view effect  = ... x ...
>    | Dull x <- view effect  = ... x ...

You forgot to define the term behaviour also. Pattern synonyms may be 
used to construct as well as to match values. Of course, I could also write

weiv :: EffectView x -> MyEffect x
weiv Bang = ...
weiv (Bing x) = ...
weiv (Dull x) = ...

or I could just add them as definitions with lower-case initials.

The pattern synonym gives me both at once, and their relationship. 
Moreover, I don't have to either (a) cover the entire type with pattern 
synonyms or (b) write a partial view; I just abbreviate whenever I find 
it convenient. More moreover, your way, the need to interpolate the view 
encoder forces me to do my matching in a guard, rather than an argument 
pattern, hence the need to say 'view effect' three times, or however 
many it happens to be, and some irritation in mixing matching on 
MyEffect x with matching on other arguments.

But perhaps constructing explicit embedding-projection pairs is a better 
solution than an abbreviation mechanism, for most people's needs.

All the best


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