:: Read a => String -> Maybe a -package:read-env-var

Parse a string using the Read instance. Succeeds if there is exactly one valid result.

>>> readMaybe "123" :: Maybe Int
Just 123

>>> readMaybe "hello" :: Maybe Int
Nothing

Safe read function. Defined here for convenience to use for parseValue.

Attempts to parse a value from the front of the string.

Parse a string using the Read instance. Succeeds if there is exactly one valid result.

>>> readMaybe "123" :: Maybe Int
Just 123

>>> readMaybe "hello" :: Maybe Int
Nothing

@since base-4.6.0.0

Reads a value encoded as a string, return Just the value or Nothing on error.

A read-based parser for the parser modifier.

A read that fails using mzero

Parse a string using the Read instance. Succeeds if there is exactly one valid result.

>>> readMaybe ("123" :: Text) :: Maybe Int
Just 123

>>> readMaybe ("hello" :: Text) :: Maybe Int
Nothing

Read in any monad.

Polymorhpic version of readMaybe.

>>> readMaybe @Int @Text "123"
Just 123

>>> readMaybe @Int @Text "aa"
Nothing

readM behaves like read, but catches failure in a monad. this allocates a 20-30 M on startup...

Parse a value to a Maybe result using a Read instance

A read with better error messages.

throws ExpectFailed. This is nice for writing your own abstractions.

A convenience function for throwing a user error. This is useful for cases where it would be too high a burden to define your own exception type. This throws an exception of type StringException. When GHC supports it (base 4.9 and GHC 8.0 and onward), it includes a call stack.