An example alternative AST construction using GADTs

AST.hs

class HasStmt a where
class HasDef a where
class HasExpr a where

data Module = Module [ParsedDef]

data ParsedDef = ParsedDef (Definition ParsedStmt) SourcePos
data ParsedStmt = ParsedStmt (Statement ParsedDef ParsedStmt ParsedExpr) SourcePos
data ParsedExpr = ParsedExpr (Expr ParsedExpr) SourcePos

instance HasDef ParsedDef where
instance HasStmt ParsedStmt where
instance HasExpr ParsedExpr where

data Definition stmt where
  VariableDef :: String -> Definition stmt
  ForeignDef  :: String -> String -> Definition stmt
  FunctionDef :: (HasStmt stmt) => String -> [Definition stmt] -> stmt -> Definition stmt

data Statement def stmt expr where
  Assign   :: (HasExpr expr) => String -> expr -> Statement def stmt expr
  If       :: (HasExpr expr, HasStmt stmt) => expr -> stmt -> Maybe stmt -> Statement def stmt expr
  While    :: (HasExpr expr, HasStmt stmt) => expr -> stmt -> Statement def stmt expr
  CallStmt :: (HasExpr expr) => expr -> Statement def stmt expr
  Return   :: (HasExpr expr) => expr -> Statement def stmt expr
  Block    :: (HasExpr expr, HasDef def, HasStmt stmt) => ([def], [stmt]) -> Statement def stmt expr

data Expr expr where
  Call       :: (HasExpr expr) => expr -> [expr] -> Expr expr
  Identifier :: String -> Expr expr
  Literal    :: Literal -> Expr expr

data Literal = Lit