any-pat

Combine multiple patterns in a single pattern.

Usage

This package ships with two QuasiQuoters: anypat and maypat. Both have the same purpose. Defining multiple possible patterns in a single clause. Indeed, consider the following example:

mightBe :: (Int, a, a) -> Maybe a
mightBe (0, a, _) = Just a
mightBe (1, _, a) = Just a
mightBe _ = Nothing

the first two clauses have some repetitive elements. We can combine the two through the anypat or maypat quasiquoter:

{-# LANGUAGE QuasiQuotes #-}
{-# LANGUAGE ViewPatterns #-}

mightBe :: (Int, a, a) -> Maybe a
mightBe [anypat|(0, a, _), (1, _, a)|] = Just a
mightBe _ = Nothing

or with maypat:

{-# LANGUAGE QuasiQuotes #-}
{-# LANGUAGE ViewPatterns #-}

mightBe :: (Int, a, a) -> Maybe a
mightBe [maypat|(0, a _), (1, _, a), _|] = a

and that's it. No, there is no need to wrap a in a Just in the last code example.

We can also use the maypat and anypat to generate expressions, with:

{-# LANGUAGE QuasiQuotes #-}

mightBe :: (Int, a, a) -> Maybe a
mightBe = [maypat|(0, a _), (1, _, a), _|]

If it is used as pattern, the ViewPatterns language extension should be enabled. Furthermore the QuasiQuotes extension should of course be enabled for any use of the quasi quoters.

The difference between the two QuasiQuoters (anypat and maypat) are the handling of variable names. Variable names defined in the pattern(s) are used in the body of the function, so it makes sense that if the clause "fires", these have a value. This thus means that a reasonable condition is that all patterns have the same set of variable names and that the variable names have the same type. The anypat requires that all patterns have the same variables, so [anypat|(0, a), (1, _)|] will raise an error: if the second pattern (1, _) would "fire" it would not provide a value for the a variable, and then we have a problem. A possible solution would be to pass a value like undefined, or an infinite loop (i.e. y = let x = x in x for example) as value, but this looks like something that would only cause a lot of trouble.

Therefore maypat comes with a different solution: it performs analysis on the variables used in the different patterns. Variables that occur in all patterns are just passed with the real value, variables that occur only in a (strict) subset of the listed patterns, are passed as a Maybe a value with Just x in case the first pattern that "fires" (left-to-right) for the value has that variable, it will be wrapped in a Just, and otherwise, it will pass Nothing as that variable.

Some functions in the base package, for example, have a simple equivalent with anypat or maypat, for example listToMaybe :: [a] -> Maybe a can be implemented as:

{-# LANGUAGE QuasiQuotes #-}

listToMaybe :: [a] -> Maybe a
listToMaybe = [maypat|(a:_), _|]

Package structure

The package has only one module: Data.Pattern.Any that exports the two QuasiQuoters named anypat and maypat together with some utility functions to obtain the variables names from a pattern.

Behind the curtains

The package transforms a sequence of patterns to a view pattern, or an expression, depending on where the quasi quoter is used. If we create a pattern [anypat|p1, p2, …, pn|], it will create a view pattern that looks like:

\case
  p1 -> Just n⃗
  p2 -> Just n⃗
  ⋮
  pn -> Just n⃗
  _ -> Nothing

with n⃗ the (sorted) tuple of names found in the patterns. It then makes a view pattern e -> n⃗ that thus maps the found values for the variables to the names that can then be used in the body of the function.

There are some (small) optimizations that for example are used if no variable names are used in the patterns, or only one. If a wildcard pattern is used, it can also omit the Maybe data type.

any-pat is inferred safe Haskell

It can not be marked safe, since the modules it depends on are not marked safe, but its safeness can be inferred by the compiler.

Contribute

You can contribute by making a pull request on the GitHub repository.

You can contact the package maintainer by sending a mail to hapytexeu+gh@gmail.com.