I wonder if someone is familiar with the Prelude´s read
function in Haskell.
The type of this function is following.
Read a => String -> a
Can someone explain me with a few examples how this function can be used and into what types the String can be cast?
Read a => String -> a
means that a
can be any type that is an instance of the Read
class. For a type to satisfy that requirement, it has to at the very least have an implementation of either of Read
's readPrec
or readsPrec
functions. Many built in types aready supply an implementation, and you can use deriving
to generate an implementation for your own custom data
types.
To specify what you want to read
the string as, you can type annotate the call directly:
read "1" :: Int
Or give the function enclosing the call to read
a signature so the compiler can figure out what you want:
myFunc :: String -> Int
myFunc s = read s
The signature says that the function returns an Int, so the compiler can infer what type to read
s
as since myFunc
returns whatever the call to read
evaluated to.