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Embedding context free grammars

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<haskell>
<haskell>
-
data Grammar b where
+
 
-
Ref :: Grammar c
+
import Maybe
-
Tok :: Char -> Grammar c
+
data Grammar a b where
-
Check :: (Char -> Bool) -> Grammar c
+
NullParser :: Grammar a b
-
Fix :: Grammar a -> Grammar c
+
Check :: (a -> Bool) -> Grammar a a
-
(:|) :: Grammar b -> Grammar b -> Grammar b
+
(:|) :: (Grammar a b) -> (Grammar a b) -> Grammar a b
-
(:&) :: Grammar b -> Grammar b -> Grammar b
+
Push :: a -> (Grammar a b) -> Grammar a b
-
Push :: Char -> Grammar a -> Grammar a
+
(:&) :: (Grammar a b) -> (Grammar a c) -> Grammar a (b,c)
-
NullParser :: Grammar b
+
FMap :: Grammar a c -> (c -> b) -> Grammar a b
 +
 
 +
 
infixl 6 :|
infixl 6 :|
-
infixl 7 :&
 
-
parse:: [Char] -> Grammar b -> Grammar b -> Bool
 
-
parse x Ref t = parse x t t
 
-
parse [c] (Tok c') _ = c == c'
 
-
parse [c] (Check y) _ = y c
 
-
parse _ (Tok _) _ = False
 
-
parse _ (Check _) _ = False
 
-
parse x (Fix g) _ = parse x g (Fix g)
 
-
parse x (g :| g') t = parse x g t || parse x g' t --cool little trick!
 
-
parse (x:xs) (g :& g') t = parse xs ((Push x g) :& g') t || (parse [] g t && parse (x:xs) g' t)
 
-
parse x (Push c g) t = parse (c:x) g t
 
-
parse _ NullParser _ = False
 
-
parse [] _ _ = False
 
-
parse' x g = parse x g NullParser
+
tok x = Check (x==)
-
</haskell>
+
-
and here's a lambda calculus parser written in this embedded language
+
parse :: [a] -> Grammar a b -> Maybe b
-
<haskell>
+
parse [c] (Check y) = if y c then Just c else Nothing
-
var = Check (\x -> x <= 'z' && x >= 'a')
+
parse x (g :| g') =
-
app = term :& term
+
let
-
term = var :| abstraction :| parenedTerm
+
r1 = parse x g
-
parenedTerm = Tok '(' :& term :& Tok ')'
+
r2 = parse x g'
-
abstraction = Tok '\\' :& var :& Tok '.' :& term
+
in
-
top = term :& Tok ';'
+
if isJust r1 then r1 else r2
-
<haskell>
+
-
let's see the results
+
parse (x:xs) (g :& g') =
-
<haskell>
+
let
-
print $ parse' "\\x.x;" top
+
r1 = parse xs ((Push x g) :& g')
-
> True
+
r2 = parse [] g
 +
r3 = parse (x:xs) g'
 +
in
 +
if isJust r1
 +
then r1
 +
else
 +
if (isJust r2) && (isJust r3)
 +
then Just (fromJust r2, fromJust r3)
 +
else Nothing
 +
parse x (Push c g) = parse (c:x) g
 +
parse x (FMap y f) = parse x y >>= f
 +
parse _ _ = Nothing
 +
 
 +
infixl 7 ~&
 +
infixl 7 ~&&
 +
infixl 7 ~&&&
 +
infixl 7 ~&&&&
 +
 
 +
(~&) = (:&)
 +
 
 +
a ~&& b = FMap
 +
(a :& b)
 +
(\((a,b),c) -> (a,b,c))
 +
 
 +
a ~&&& b = FMap
 +
(a :& b)
 +
(\((a,b,c),d) -> (a,b,c,d))
 +
 
 +
a ~&&&& b = FMap
 +
(a :& b)
 +
(\((a,b,c,d),e) -> (a,b,c,d,e))
 +
 
 +
a ~&&&&& b = FMap
 +
(a :& b)
 +
(\((a,b,c,d,e),f) -> (a,b,c,d,e,f))
</haskell>
</haskell>
-
Let's check out a recursive grammar of all strings containing only c's of length at least 1
+
and here's a lambda calculus parser
<haskell>
<haskell>
-
top' = Fix ((Tok 'c' :& Ref) :| Tok 'c')
+
 
-
print $ parse' "cccccc" top'
+
 
-
> True
+
data Term = Var Char | App Term Term | Abs Char Term deriving Show
 +
 
 +
var =
 +
(Check (\x -> x <= 'z' && x >= 'a'))
 +
 +
app = term ~& term
 +
 
 +
term = FMap var Var :| abstraction :| parenedTerm
 +
 
 +
parenedTerm = FMap
 +
(tok '(' ~& term ~&& tok ')')
 +
(\(a,b,c) -> b)
 +
 +
abstraction = FMap
 +
(tok '\\' ~& var ~&& tok '.' ~&&& term)
 +
(\(a,b,c,d) -> Abs b d)
 +
top = FMap
 +
(term ~& tok ';')
 +
fst
 +
 
 +
main = print $ parse "\\x.x;" top
 +
 
</haskell>
</haskell>
 +
 +
[[Category:Code]]

Current revision

Here's how to embed a context free grammar parser into haskell: 

import Maybe
data Grammar a b where
    NullParser ::  Grammar a b
    Check :: (a -> Bool) -> Grammar a a
    (:|) :: (Grammar  a b) -> (Grammar  a b) -> Grammar a b
    Push :: a ->  (Grammar  a b) -> Grammar a b
    (:&) :: (Grammar  a b) -> (Grammar  a c) -> Grammar a (b,c)
    FMap :: Grammar  a c -> (c -> b) -> Grammar  a b
 
 
infixl 6 :|
 
 
 
tok x = Check (x==)
 
parse :: [a] -> Grammar a b -> Maybe b
parse [c] (Check y) = if y c then Just c else Nothing
parse x (g :| g') = 
    let 
        r1 = parse x g 
        r2 = parse x g' 
    in
        if isJust r1 then r1 else r2
 
parse (x:xs) (g :& g') = 
     let 
         r1 = parse xs ((Push x g) :& g') 
         r2 = parse [] g 
         r3 = parse (x:xs) g'
     in
       if isJust r1 
       then r1 
       else 
           if (isJust r2) && (isJust r3) 
           then Just (fromJust r2, fromJust r3)
           else Nothing
parse x (Push c g) = parse (c:x) g
parse x (FMap y f) = parse x y >>= f
parse _ _ = Nothing
 
infixl 7 ~&
infixl 7 ~&&
infixl 7 ~&&&
infixl 7 ~&&&&
 
(~&) = (:&)
 
a ~&& b = FMap 
          (a :& b)
          (\((a,b),c) -> (a,b,c))
 
a ~&&& b = FMap 
           (a :& b)
           (\((a,b,c),d) -> (a,b,c,d))
 
a ~&&&& b = FMap 
            (a :& b)
            (\((a,b,c,d),e) -> (a,b,c,d,e))
 
a ~&&&&& b = FMap 
             (a :& b)
             (\((a,b,c,d,e),f) -> (a,b,c,d,e,f))

and here's a lambda calculus parser 

data Term = Var Char | App Term Term | Abs Char Term deriving Show
 
var = 
    (Check (\x -> x <= 'z' && x >= 'a'))
 
app = term ~& term
 
term = FMap var Var :| abstraction :| parenedTerm  
 
parenedTerm = FMap 
              (tok '(' ~& term ~&& tok ')')
              (\(a,b,c) -> b)
 
abstraction = FMap 
              (tok '\\' ~& var ~&& tok '.' ~&&& term)
              (\(a,b,c,d) -> Abs b d)
top = FMap 
      (term ~& tok ';')
      fst
 
main = print $ parse "\\x.x;" top
Retrieved from "http://www.haskell.org/haskellwiki/Embedding_context_free_grammars"

Category: Code