System.Console.Wizard

A Wizard b a is a conversation with the user via back-end b that will result in a data type a, or may fail. A Wizard is made up of one or more "primitives" (see below), composed using the Applicative, Monad and Alternative instances. The Alternative instance is, as you might expect, a maybe-style cascade. If the first wizard fails, the next one is tried. mzero can be used to induce failure directly.

The Wizard constructor is exported here for use when developing backends, but it is better for end-users to simply pretend that Wizard is an opaque data type. Don't depend on this unless you have no other choice.

Wizards are, internally, just a maybe transformer over a free monad built from some coproduct of functors, each of which is a primitive action.

A string for a prompt

Run a wizard using some back-end.

Subsumption of two functors. You shouldn't define any of your own instances of this when writing back-ends, rely only on GeneralizedNewtypeDeriving.

Coproduct of two functors

Read one line of input from the user. Cannot fail (but may throw exceptions, depending on the backend).

Read one line of input, with some default text already present, before and/or after the editing cursor.

Read one line of password input, with an optional mask character.

Read a single character only from input. Cannot fail (but may throw exceptions, depending on the backend).

Output a string. Does not fail.

Output a string followed by a newline. Does not fail.

Retry produces a wizard that will retry the entire conversation again if it fails. It is simply retry x = x <|> retry x.

Same as retry, except an error message can be specified.

x `defaultTo` y will return y if x fails, e.g parseRead line `defaultTo` 0.

Like fmap, except the function may be partial (Nothing causes the wizard to fail).

validator p causes a wizard to fail if the output value does not satisfy the predicate p.

Simply validator (not . null), makes a wizard fail if it gets an empty string.

Makes a wizard fail if it gets an ordered quantity outside of the given range.

Simply parser readP. Attaches a simple read parser to a Wizard.

Translate a maybe value into wizard success/failure.

Ensures that a maybe value satisfies a given predicate.

A read-based parser for the parser modifier.