ref: c9fd045412e8361977ed3dd53cb7aa627242a00c
dir: /lib/Text/Read.hs/
-- Copyright 2023,2024 Lennart Augustsson
-- See LICENSE file for full license.
-- Temporary Read class
module Text.Read(
ReadS,
Read(..),
read,
reads,
readMaybe,
readParen,
) where
import Primitives
import Control.Error
import Control.Monad
import Control.Monad.Fail
import Data.Char
import Data.Bool_Type
import Data.Eq
import Data.List_Type
import Data.Maybe_Type
import Data.Num
import Data.Int
import Text.Read_Class
read :: forall a . Read a => String -> a
read s =
case readsPrec 0 s of
[(a, [])] -> a
_ -> error "read: failed"
reads :: forall a . Read a => ReadS a
reads = readsPrec 0
readMaybe :: forall a . Read a => String -> Maybe a
readMaybe s =
case readsPrec 0 s of
[(a, [])] -> Just a
_ -> Nothing
readParen :: forall a . Bool -> ReadS a -> ReadS a
readParen b g =
if b then mandatory else optional
where
optional :: ReadS a
optional r = g r ++ mandatory r
mandatory :: ReadS a
mandatory r = do
('(',s) <- lex r
(x,t) <- optional s
(')',u) <- lex t
return (x,u)
-- Really bad lexer
lex :: ReadS Char
lex "" = []
lex (c:cs) | isSpace c = lex cs
| True = [(c, cs)]
-------------------------------------------------------
-- To avoid circular imports, some instances go here.
-- XXX make this better
instance Read Int where
readsPrec _ ccs@(c:cs) | isDigit c = loop False 0 ccs
where
loop neg res (c:cs) | isDigit c = loop neg (res * (10::Int) + ord c - ord '0') cs
loop neg res s = [(if neg then -res else res, s)]
readsPrec _ _ = []