Why is Haskell unable to read "7e7" but able to read "7a7"?

Question

Try to do:

Prelude> reads "7a7" :: [(Int, String)]
[(7,"a7")]

Prelude> reads "7e7" :: [(Int, String)]
[]

I tested this for all possible characters in the middle. They all work except for 'e'. It seems as if Haskell tries to interpret the number in scientific notation, but it can't because I'm asking for Int.

It seems like a bug to me.

Answer 1

GHC is indeed buggy. Its implementation of Numeric.readSigned uses the following:

read'' r = do
    (str,s) <- lex r
    (n,"")  <- readPos str
    return (n,s)

The lex call will try to parse any lexeme, and this means that for "7e7" it yields [("7e7", "")], because "7e7" is a whole lexeme for a floating point literal. Then it tries to get a complete parse out of readPos, which in this case is an argument for which Numeric.readDec was passed in, and readDec will yield, correctly, [(7, "e7")] for the string "7e7". That fails pattern matching against (n, ""), and ends up as [].

I think it should be simply as follows:

read'' = readPos

Answer 2

7e7 :: Fractional a => a so it can't be read as an Int, but it can be read as a Float or Double.

ghci> :t 7e7
7e7 :: Fractional a => a