-- | This module contains definitions for plural form selection expressions AST, -- and an evaluator function for such expressions. -- module Data.Gettext.Plural ( -- * Data types BinOp (..), Expr (..), -- * Expressions eval ) where import Data.Bits (xor) -- | Supported binary operations data BinOp = Equals | NotEquals | Greater | NotGreater | Less | NotLess | And | Or | Xor | Mod | Plus | Minus | Multiply | Divide deriving (Eq, Show) -- | Plural form selection expression AST data Expr = N -- ^ The @n@ variable | Literal Int -- ^ Literal number | If Expr Expr Expr -- ^ Ternary operator (... ? ... : ...) | Negate Expr -- ^ Unary arithmetic negation (as in @-1@). | Not Expr -- ^ Unary logic negation (as in @! (n == 1)@) | Binary BinOp Expr Expr -- ^ Binary operation deriving (Eq, Show) order :: (Int -> Int -> Bool) -> (Int -> Int -> Int) order op x y = if op x y then 1 else 0 logic :: (Bool -> Bool -> Bool) -> (Int -> Int -> Int) logic op x y = if op (x /= 0) (y /= 0) then 1 else 0 evalOp :: BinOp -> (Int -> Int -> Int) evalOp Equals = order (==) evalOp NotEquals = order (/=) evalOp Greater = order (>) evalOp NotGreater = order (<=) evalOp Less = order (<) evalOp NotLess = order (>=) evalOp And = logic (&&) evalOp Or = logic (||) evalOp Xor = logic xor evalOp Mod = mod evalOp Plus = (+) evalOp Minus = (-) evalOp Multiply = (*) evalOp Divide = \x y -> if y == 0 then error "Division by zero in plural form selection expression" else x `div` y -- | Evaluate the expression eval :: Expr -- ^ Expression -> Int -- ^ Number -> Int -- ^ Plural form index defined by expression eval N n = n eval (Literal x) _ = x eval (If cond true false) n = if eval cond n /= 0 then eval true n else eval false n eval (Binary op x y) n = evalOp op (eval x n) (eval y n) eval (Negate x) n = negate \$ eval x n