haskell - Haskell 中科学记数法的阅读更方便

Question

Haskell 的 read 对 float 有点过于严格:

$ ghci
GHCi, version 8.2.2: http://www.haskell.org/ghc/  :? for help
Prelude> read "-1E34" :: Double
-1.0e34
Prelude> read "-1.E34" :: Double
*** Exception: Prelude.read: no parse
Prelude>

是否有接受第二种形式的 read 版本？这在物理科学中很常见。例如，Fortran 可以读取和写入此类形式。

Haskell 不支持的另一个例子是“0.1”的“.1”。这种情况就更常见了。我只是不想转换输入的 ascii 文件。 。 。 .

Answer 1

这是一个执行此操作的自定义解析器，使用 megaparsec .

import Text.Megaparsec
import Text.Megaparsec.Char

realLiteral :: (MonadParsec e s m, Token s ~ Char) => m Double
realLiteral = mkFloat <$> sign <*> intgPart <*> fracPart <*> exponent
 where mkFloat sgn itg frc expn
           = fromIntegral sgn * (fromIntegral itg + frc) * 10^^expn
       sign = (-1) <$ char '-'
           <|> 1   <$ char '+'
           <|> pure 1
       intgPart = read . ('0':) <$> many digitChar
       fracPart = char '.' *> (toFrc<$>many digitChar)
               <|> pure 0
        where toFrc "" = 0
              toFrc digits = read digits / 10^length digits
       exponent = oneOf "eEdD" *> ((*) <$> sign <*> (read<$>some digitChar))
               <|> pure 0

[1 of 1] Compiling Main             ( wtmpf-file5764.hs, interpreted )
Ok, 1 module loaded.
*Main> parseMaybe realLiteral "1"
Just 1.0
*Main> parseMaybe realLiteral "-3"
Just (-3.0)
*Main> parseMaybe realLiteral "-9e+2"
Just (-900.0)
*Main> parseMaybe realLiteral ".3e+9"
Just 3.0e8
*Main> parseMaybe realLiteral "-1.E34"
Just (-1.0000000000000001e34)
*Main> parseMaybe realLiteral "-1.673986e-40"
Just (-1.6739859999999999e-40)
*Main> parseMaybe realLiteral "-3.E+16"
Just (-3.0e16)