by Phil Freeman on 2013/10/27

I'd like to show a neat use of arrows for pretty printing an AST.

```
{-# LANGUAGE GADTs, GeneralizedNewtypeDeriving #-}
import Data.Maybe (fromMaybe)
import Data.Function (fix)
import qualified Control.Category as C
import Control.Category ((>>>))
import qualified Control.Arrow as A
import Control.Arrow ((***), (<+>))
```

Suppose you had defined a type of syntax trees and wanted to write a function to print their representations as code:

```
data Expr = Var String
| Abs String Expr
| App Expr Expr deriving Show
```

A first attempt might look something like this:

```
pretty1 :: Expr -> String
pretty1 (Var v) = v
pretty1 (Abs v e) = "\\" ++ v ++ " -> " ++ pretty1 e
pretty1 (App e1 e2) = "(" ++ pretty1 e1 ++ ") (" ++ pretty1 e2 ++ ")"
```

This certainly generates valid code, but the resulting strings tend to contain a lot of redundant parentheses:

```
ghci> let s = Abs "x" $
Abs "y" $
Abs "z" $
App (App (Var "x") (Var "z"))
(App (Var "y") (Var "z"))
ghci> pretty1 s
"\\x -> \\y -> \\z -> ((x) (z)) ((y) (z))"
```

Another approach is to thread the current precedence level as an argument, and to parenthesize as a last resort:

```
type Precedence = Int
pretty2 :: Expr -> String
pretty2 = pretty2' 0
where
pretty2' :: Precedence -> Expr -> String
pretty2' _ (Var v) = v
pretty2' p (Abs v e) | p < 2 = "\\" ++ v ++ " -> " ++ pretty2' 0 e
pretty2' p (App e1 e2) = pretty2' 1 e1 ++ " " ++ pretty2' p e2
pretty2' _ e = "(" ++ pretty2' 0 e ++ ")"
```

We can verify that this approach generates better code, and that the precedence rules are still respected:

```
ghci> pretty2 s
"\\x -> \\y -> \\z -> x z (y z)"
ghci> let k = App (Abs "x" $ Var "x") (Abs "x" $ Var "x")
ghci> pretty2 k
"(\\x -> x) (\\x -> x)"
```

These approaches are fine, but as the complexity of the AST type increases, I find it harder to keep the various precedence relationships in mind.

Arrows provide a way to express pattern matches as first class values in a simple way, and then to compose those patterns to create full pretty printers.

First class patterns and their use in pretty printing are not new ideas: interested readers might like to take a look at the following papers for more information:

- "Type-safe pattern combinators (Functional Pearl)" by Morten Rhiger
- "Invertible Syntax Descriptions: Unifying Parsing and Pretty Printing" by Tillmann Rendel and Klaus Ostermann

However, I think the use of arrows provides a novel way to build up patterns into complex pretty printers.

The really neat thing is that almost all of the required code can be derived using `GeneralizedNewtypeDeriving`

!

Here is the definition of a `Pattern`

as an `Arrow`

. It takes a value of type `a`

, and either matches successfully, returning a value of type `b`

, or fails. Failure is modelled using the Kleisli category for the `Maybe`

monad:

```
newtype Pattern a b = Pattern { runPattern :: A.Kleisli Maybe a b }
deriving (C.Category, A.Arrow, A.ArrowZero, A.ArrowPlus)
pattern :: Pattern a b -> a -> Maybe b
pattern = A.runKleisli . runPattern
```

We can derive instances for `Category`

, `Arrow`

, `ArrowZero`

, and `ArrowPlus`

. The intuition here is that `Category`

gives us composition of patterns, i.e. nested patterns, `Arrow`

gives combinators for working with patterns involving tuples, and `ArrowZero`

and `ArrowPlus`

give us a way to deal with failure and backtracking.

Note: there is also an instance for `Applicative`

which gives another way to work with simultaneous patterns, but I won't write it out here.

One thing we can't immediately `derive`

is the `Functor`

instance for `Pattern`

, which will come in useful later. Fortunately, it is easy to write by hand

```
instance Functor (Pattern a) where
fmap f p = Pattern $ A.Kleisli $ fmap f . pattern p
```

Here are some examples of `Pattern`

s

```
var :: Pattern Expr String
var = Pattern $ A.Kleisli var'
where var' (Var s) = Just s
var' _ = Nothing
lam :: Pattern Expr (String, Expr)
lam = Pattern $ A.Kleisli abs'
where abs' (Abs s e) = Just (s, e)
abs' _ = Nothing
app :: Pattern Expr (Expr, Expr)
app = Pattern $ A.Kleisli app'
where app' (App e1 e2) = Just (e1, e2)
app' _ = Nothing
```

I imagine these are the sort of the thing one could write a Template Haskell splice for. They also seem quite similar to Prisms, which might provide another way to write this code.

Now we can write some combinators in the spirit of `Text.Parsec`

which allow us to build up new patterns from old, and to apply a pattern recursively:

```
chainl :: Pattern a (a, a) -> (r -> r -> r) -> Pattern a r -> Pattern a r
chainl split f p = fix $ \c -> (split >>> ((c <+> p) *** p) >>> A.arr (uncurry f))
chainr :: Pattern a (a, a) -> (r -> r -> r) -> Pattern a r -> Pattern a r
chainr split f p = fix $ \c -> (split >>> (p *** (c <+> p)) >>> A.arr (uncurry f))
wrap :: Pattern a (s, a) -> (s -> r -> r) -> Pattern a r -> Pattern a r
wrap split f p = fix $ \c -> (split >>> (C.id *** (c <+> p)) >>> A.arr (uncurry f))
```

In fact, we can go one step further and derive a pattern from a precedence table in the manner of `Text.Parsec.Expr`

:

```
data OperatorTable a r = OperatorTable { runOperatorTable :: [ [Operator a r] ] }
data Operator a r where
AssocL :: Pattern a (a, a) -> (r -> r -> r) -> Operator a r
AssocR :: Pattern a (a, a) -> (r -> r -> r) -> Operator a r
Wrap :: Pattern a (s, a) -> (s -> r -> r) -> Operator a r
buildPrettyPrinter :: OperatorTable a r -> Pattern a r -> Pattern a r
buildPrettyPrinter table p = foldl (\p' ops -> foldl1 (<+>) (flip map ops $ \op ->
case op of
AssocL pat g -> chainl pat g p'
AssocR pat g -> chainr pat g p'
Wrap pat g -> wrap pat g p'
) <+> p') p $ runOperatorTable table
```

We need one final function, which parenthesizes an expression:

```
parenthesize :: Pattern a String -> Pattern a String
parenthesize = fmap parens
where
parens s = '(':s ++ ")"
```

This gives us the parts we need to express our previous pretty printer as a `Pattern`

:

```
expr = buildPrettyPrinter ops (var <+> parenthesize expr)
where
ops = OperatorTable
[ [ AssocL app $ \e1 e2 -> e1 ++ " " ++ e2 ]
, [ Wrap lam $ \b s -> "\\" ++ b ++ " -> " ++ s ]
]
pattern3 :: Expr -> String
pattern3 = fromMaybe (error "Incomplete pattern match") . pattern expr
```

Note that, just like when we define parsers using `Text.Parsec.Expr`

, the use of combinators allows us to write code which directly represents the precedence table!

Here's another example, of expressions supporting integer constants and binary operators.

```
data Eqn = Const Int
| Bin Eqn Char Eqn deriving Show
con :: Pattern Eqn Int
con = Pattern $ A.Kleisli con'
where con' (Const n) = Just n
con' _ = Nothing
bin :: Char -> Pattern Eqn (Eqn, Eqn)
bin c = Pattern $ A.Kleisli bin'
where bin' (Bin e1 c' e2) | c == c' = Just (e1, e2)
bin' _ = Nothing
eqn = buildPrettyPrinter ops (fmap show con <+> parenthesize eqn)
where
ops = OperatorTable
[ [ binOp '*', binOp '/' ]
, [ binOp '+', binOp '-' ]
]
binOp c = AssocL (bin c) $ \e1 e2 -> e1 ++ c : e2
```

For yet another (more developed) example, see my current project on GitHub, here.

Copyright Phil Freeman 2010-2015