Read – readable types

The Read class supports reading and conversion of string representations of values into actual types using the read method. All the basic types of Haskell are also instances of the Read class. Note that we sometimes have to specify the type to be read in cases where the intended type cannot be inferred:

ghci> read "False"
*** Exception: Prelude.read: no parse

ghci> read "False" :: Bool
False

That is, in the first case, the compiler does not know whether to read "False" as a type of String or Bool, so it throws an exception, while in the second case, we explicitly state that we want to read it as a Bool. However, if we had some additional boolean function (e.g. negation) to be called on the read argument, the compiler would be able to automatically infer the wanted type:

ghci> not $ read "False"
True