Version 30 (modified by goldfire, 4 years ago) (diff)

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# Pattern Synonyms

Most language entities in Haskell can be named so that they can be abbreviated instead of written out in full. This proposal provides the same power for patterns.

See the implementation page for implementation details.

Relevant closed tickets:

Relevant open tickets:

## Motivating example

Here is a simple representation of types

```    data Type = App String [Type]
```

Using this representations the arrow type looks like `App "->" [t1, t2]`. Here are functions that collect all argument types of nested arrows and recognize the `Int` type:

```   collectArgs :: Type -> [Type]
collectArgs (App "->" [t1, t2]) = t1 : collectArgs t2
collectArgs _ = []

isInt (App "Int" []) = True
isInt _ = False
```

Matching on `App` directly is both hard to read and error prone to write.

The proposal is to introduce a way to give patterns names:

```   pattern Arrow t1 t2 = App "->" [t1, t2]
pattern Int = App "Int" []
```

And now we can write

```   collectArgs :: Type -> [Type]
collectArgs (Arrow t1 t2) = t1 : collectArgs t2
collectArgs _ = []

isInt Int = True
isInt _ = False
```

Here is a second example from pigworker on Reddit. Your basic sums-of-products functors can be built from this kit.

```newtype K a        x  = K a
newtype I          x  = I x
newtype (:+:) f g  x  = Sum (Either (f x) (g x))
newtype (:*:) f g  x  = Prod (f x, g x)
```

and then you can make recursive datatypes via

```newtype Fix f = In (f (Fix f))
```

e.g.,

```type Tree = Fix (K () :+: (I :*: I))
```

and you can get useful generic operations cheaply because the functors in the kit are all `Traversable`, admit a partial zip operation, etc.

You can define friendly constructors for use in expressions

```leaf :: Tree
leaf = In (Sum (Left (K ())))
node :: Tree -> Tree -> Tree
node l r = In (Sum (Right (Prod (I l, I r))))
```

but any `Tree`-specific pattern matching code you write will be wide and obscure. Turning these definitions into pattern synonyms means you can have both readable type-specific programs and handy generics without marshalling your data between views.

## Uni-directional (pattern-only) synonyms

The simplest form of pattern synonyms is the one from the examples above. The grammar rule is:

`pattern` conid varid1 ... varidn `<-` pat

`pattern` varid1 consym varid2 `<-` pat

• Each of the variables on the left hand side must occur exactly once on the right hand side
• Pattern synonyms are not allowed to be recursive. Cf. type synonyms.
TODO

There have been several proposals for the syntax of defining pattern-only synonyms:

• `pattern` conid varid1 ... varidn `~` pat
• `pattern` conid varid1 ... varidn `:=` pat
• `pattern` conid varid1 ... varidn `->` pat
• `pattern` conid varid1 ... varidn `<-` pat

Pattern synonyms can be exported and imported by prefixing the conid with the keyword `pattern`:

```   module Foo (pattern Arrow) where ...
```

This is required because pattern synonyms are in the namespace of constructors, so it's perfectly valid to have

```   data P = C
pattern P = 42
```

You may also give a type signature for a pattern, but as with most other type signatures in Haskell it is optional:

`pattern` conid `::` type

E.g.

```   pattern Arrow :: Type -> Type -> Type
pattern Arrow t1 t2 <- App "->" [t1, t2]
```

Together with ViewPatterns we can now create patterns that look like regular patterns to match on existing (perhaps abstract) types in new ways:

```import qualified Data.Sequence as Seq

pattern Empty <- (Seq.viewl -> Seq.EmptyL)
pattern x :< xs <- (Seq.viewl -> x Seq.:< xs)
pattern xs :> x <- (Seq.viewr -> xs Seq.:> x)
```

## Simply-bidirectional pattern synonyms

In cases where pat is in the intersection of the grammars for patterns and expressions (i.e. is valid both as an expression and a pattern), the pattern synonym can be made bidirectional, and can be used in expression contexts as well. Bidirectional pattern synonyms have the following syntax:

`pattern` conid varid1 ... varidn `=` pat

`pattern` varid1 consym varid2 `=` pat

For example, the following two pattern synonym definitions are rejected, because they are not bidirectional (but they would be valid as pattern-only synonyms)

```   pattern ThirdElem x = _:_:x:_
pattern Snd y = (x, y)
```

since the right-hand side is not a closed expression of {x} and {y} respectively.

In contrast, the pattern synonyms for Arrow and Int above are bidirectional, so you can e.g. write:

```   arrows :: [Type] -> Type -> Type
arrows = flip \$ foldr Arrow
```

## Explicitly-bidirectional pattern synonyms

What if you want to use `Succ` in an expression:

```    pattern Succ n <- n1 | let n = n1 -1, n >= 0
```

It's clearly impossible since its expansion is a pattern that has no meaning as an expression. Nevertheless, if we want to make what looks like a constructor for a type we will often want to use it in both patterns and expressions. This is the rationale for the most complicated synonyms, the bidirectional ones. They provide two expansions, one for patterns and one for expressions.

`pattern` conid varid1 ... varidn `<-` pat `where` cfunlhs rhs

where cfunlhs is like funlhs, except that the functions symbol is a conid instead of a varid.

Example:

```   pattern Succ n <- n1 | let n = n1-1, n >= 0 where
Succ n = n + 1
```

TODO: Rewrite this example to not use ViewPatternsAlternative

The first part as is before and describes the expansion of the synonym in patterns. The second part describes the expansion in expressions.

```   fac 0 = 0
fac (Succ n) = Succ n * fac n
```

## Associated pattern synonyms

Just like data types and type synonyms can be part of a class declaration, it would be possible to have pattern synonyms as well.

Example:

```   class ListLike l where
pattern Nil :: l a
pattern Cons :: a -> l a -> a
isNil :: l a -> Bool
isNil Nil = True
isNil (Cons _ _) = False
append :: l a -> l a -> l a

instance ListLike [] where
pattern Nil = []
pattern Cons x xs = x:xs
append = (++)

headOf :: (ListLike l) => l a -> Maybe a
headOf (Cons x _) = Just x
```

One could go one step further and leave out the `pattern` keyword to obtain associated constructors, which are required to be bidirectional. The capitalized identifier would indicate that a pattern synonym is being defined. For complicated cases one could resort to the `where` syntax (shown above).

TODO: Syntax for associated pattern synonym declarations to discern between pattern-only and bidirectional pattern synonyms

## Static semantics

A unidirectional pattern synonym declaration has the form

```pattern P var1 var2 ... varN <- pat
```

The formal pattern synonym arguments `var1`, `var2`, ..., `varN` are brought into scope by the pattern pat on the right-hand side. The declaration brings the name `P` as a pattern synonym into the module-level scope.

The pattern synonym `P` is assigned a pattern type of the form

```pattern CProv => P t1 t2 ... tN :: CReq => t
```

where `t` is a simple type with no context, and `CReq` and `CProv` are type contexts.

A pattern synonym of this type can be used in a pattern if the instatiated (monomorphic) type satisfies the constraints of `CReq`. In this case, it extends the context available in the right-hand side of the match with `CProv`, just like how an existentially-typed data constructor can extend the context.

For example, in the following definition:

```{-# LANGUAGE PatternSynonyms, GADTs, ViewPatterns #-}
module ShouldCompile where

data T a where
MkT :: (Eq b) => a -> b -> T a

f :: (Show a) => a -> Bool

pattern P x <- MkT (f -> True) x
```

the pattern type of `P` is

```pattern (Eq b) => P b :: (Show a) => T a
```

A bidirectional pattern synonym declaration has the form

```pattern P var1 var2 ... varN = pat
```

where both of the following are well-typed declarations:

```pattern P1 var1 var2 ... varN <- pat

P2 = \var1 var2 ... varN -> pat
```

In this case, the pattern type of `P` is simply the pattern type of `P1`, and its expression type is the type of `P2`. The name `P` is brought into the module-level scope both as a pattern synonym and as an expression.

## Dynamic semantics

A pattern synonym occurance in a pattern is evaluated by first matching against the pattern synonym itself, and then on the argument patterns. For example, given the following definitions:

```pattern P x y <- [x, y]

f (P True True) = True
f _             = False

g [True, True] = True
g _            = False
```

the behaviour of `f` is the same as

```f [x, y] | True <- x, True <- y = True
f _                             = False
```

Because of this, the eagerness of `f` and `g` differ:

```*Main> f (False:undefined)
*** Exception: Prelude.undefined
*Main> g (False:undefined)
False
```

## Typed pattern synonyms

So far patterns only had syntactic meaning. In comparison Ωmega has typed pattern synonyms, so they become first class values. (I am not suggesting this for Haskell, yet.)