Fix parsing of constants (no longer allow empty string or uppercase start).

Add a parser for conjunctive queries.
Add some prettyprinters for the syntax.

CHANGELOG.md

@@ -4,4 +4,4 @@
 
 * We have a basic parser and a very naive evaluator for datalog programs that can extend a database with all the facts which are immediate
   consequences of the rules and the current (initially empty) relations, which can be iterated to a fixed point to produce all consequences.
-
+* Added some prettyprinters and a separate parser for conjunctive queries which we needed for the web frontend.

src/Datalog/Parser.hs

@@ -2,10 +2,12 @@ module Datalog.Parser
     ( parseTerm
     , parseAtom
     , parseRule
+    , parseQuery
     , parseProgram
     )
   where
 
+import Control.Applicative (liftA2)
 import Datalog.Syntax
 import Data.Text (Text)
 import Data.Text qualified as T
@@ -36,7 +38,7 @@ comma = () <$ symbol ","
 period = () <$ symbol "."
 
 parseCon :: (MonadParsec e Text m) => m Term
-parseCon = Con . ConId . T.pack <$> lexeme (many alphaNumChar)
+parseCon = Con . ConId . T.pack <$> lexeme (liftA2 (:) (numberChar <|> lowerChar) (many alphaNumChar))
 
 parseVar :: (MonadParsec e Text m) => m Term
 parseVar = Var . VarId . T.pack <$> lexeme (liftA2 (:) upperChar (many alphaNumChar))
@@ -50,6 +52,9 @@ parseAtom = do
   args <- parens (parseTerm `sepBy` comma)
   return (Atom rel args)
 
+parseQuery :: Parser [Atom]
+parseQuery = parseAtom `sepBy` comma
+
 parseFact :: Parser Rule
 parseFact = do
   headAtom <- parseAtom
@@ -59,7 +64,7 @@ parseFact = do
 parseRule :: Parser Rule
 parseRule = try parseFact <|> do
   headAtom <- parseAtom <* symbol ":-"
-  bodyAtoms <- parseAtom `sepBy` comma
+  bodyAtoms <- parseQuery
   period
   return (headAtom :- bodyAtoms)
 

src/Datalog/Syntax.hs

@@ -58,3 +58,29 @@ instance HasConstants Rule where
 
 instance HasConstants Program where
   constants (Program rs) = constants rs
+
+class Pretty t where
+  pretty :: t -> Text
+
+instance Pretty ConId where
+  pretty (ConId t) = t
+
+instance Pretty VarId where
+  pretty (VarId t) = t
+
+instance Pretty RelId where
+  pretty (RelId t) = t
+
+instance Pretty Term where
+  pretty (Con conId) = pretty conId
+  pretty (Var varId) = pretty varId
+
+instance Pretty Atom where
+  pretty (Atom relId terms) = pretty relId <> "(" <> T.intercalate "," (map pretty terms) <> ")"
+
+instance Pretty Rule where
+  pretty (h :- []) = pretty h <> "."
+  pretty (h :- ts) = pretty h <> " :- " <> T.intercalate ", " (map pretty ts) <> "."
+
+instance Pretty Program where
+  pretty (Program rs) = T.unlines (map pretty rs)