Conversion of values to Bool.

Instances of ToBool that are also Boolean should obey the following laws:

p || q = if toBool p then true else q

p && q = if toBool p then q else false

A simple conditional operator

if' with the Bool argument at the end (infixr 1).

A catamorphism (aka fold) for booleans. This is analogous to foldr, maybe, and either. The first argument is the false case, the second argument is the true case, and the last argument is the predicate value.

if' lifted to Monad. Unlike liftM3 if', this is short-circuiting in the monad, such that only the predicate action and one of the remaining argument actions are executed.

Lifted inclusive disjunction. Unlike liftM2 (||), This function is short-circuiting in the monad. Fixity is the same as || (infixr 2).

Lifted conjunction. Unlike liftM2 (&&), this function is short-circuiting in the monad. Fixity is the same as && (infxr 3).

Lifted boolean negation.

Lifted boolean exclusive disjunction.

Lisp-style conditionals. If no conditions match, then a runtime exception is thrown. Here's a trivial example:

  signum x = cond [(x > 0     , 1 )
                  ,(x < 0     , -1)
                  ,(otherwise , 0 )]

Analogous to the cond function with a default value supplied, which will be used when no condition in the list is matched.

Lisp-style conditionals generalized over MonadPlus. If no conditions match, then the result is mzero. This is a safer variant of cond.

Here's a highly contrived example using fromMaybe:

  signum x = fromMaybe 0 . condPlus $ [(x > 0, 1 ) 
                                      ,(x < 0, -1)]

Alternatively, you could use the <| operator from Hoare's ternary conditional choice operator, like so:

  signum x = 0 <| condPlus [(x > 0, 1 ) 
                           ,(x < 0, -1)]

cond lifted to Monad. If no conditions match, a runtime exception is thrown.

condPlus lifted to Monad. If no conditions match, then mzero is returned.

A synonym for return true.

Conditional composition. If the predicate is False, id is returned instead of the second argument. This function, for example, can be used to conditionally add functions to a composition chain.

Conditional monoid operator. If the predicate is False, the second argument is replaced with mempty. The fixity of this operator is one level higher than <>.

It can also be used to chain multiple predicates together, like this:

even (length ls) ?<> not (null ls) ?<> ls

Composes a predicate function and 2 functions into a single function. The first function is called when the predicate yields True, the second when the predicate yields False.

Note that after importing Control.Monad.Instances, select becomes a special case of ifM.

select lifted to Monad.

An operator that allows you to write C-style ternary conditionals of the form:

p ? t ?? f

Note that parentheses are required in order to chain sequences of conditionals together. This is probably a good thing.

Right bracket of the conditional choice operator. If the predicate is True, returns Nothing, otherwise it returns Just the right-hand argument.

Left bracket of the conditional choice operator. This is equivalent to fromMaybe

A monadic variant of |>.

A monadic variant of <|.

Unicode rebinding of |>.

Unicode rebinding of <|.

Generalization of guard

A variant of guard with a monadic predicate.

Generalization of when

A variant of when with a monadic predicate.

Generalization of unless

A variant of unless with a monadic predicate.