Error

Prelude Text.Regex.Applicative
λ> :set -XOverloadedStrings
Prelude Text.Regex.Applicative
λ> let number = "one" *> pure 1  <|>  "two" *> pure 2

<interactive>:7:5:
    Could not deduce (Data.String.IsString (f a0))
    from the context (Num a,
                      Data.String.IsString (f a2),
                      Data.String.IsString (f a3),
                      Alternative f)
      bound by the inferred type for ‘number’:
                 (Num a, Data.String.IsString (f a2), Data.String.IsString (f a3),
                  Alternative f) =>
                 f a
      at <interactive>:7:5-50
    The type variable ‘a0’ is ambiguous
    When checking that ‘number’ has the inferred type
      number :: forall (f :: * -> *) a a1 a2.
                (Num a, Data.String.IsString (f a1), Data.String.IsString (f a2),
                 Alternative f) =>
                f a
    Probable cause: the inferred type is ambiguous