LanguageExt.Core ► Monads ► Monadic conditionals

guard, when, and unless allow for conditional operations and short-cutting in monadic expressions.

Guards

Guards are used to stop the monadic expression continuing if a flag is true (for guard) or false (for guardnot).

They only work with monads that have an 'alternative value' (which is usually used as the error condition: Left in Either for example). An alternative value is provided when the guard triggers:

from x in ma
from _ in guard(x == 100, Error.New("x should be 100"))
select x;

Supported monads are:

Either
EitherUnsafe
EitherAsync
Fin
Validation
Aff
Eff

When and Unless

when and unless are similar to guards, but instead of providing the alternative value, you provide an alternative monad to run. This monad could be in a failed state, or it could run a successful side effect (an Aff calling Console<RT>.writeLine() for example).

from x in ma
from _ in when(x == 100, Console.writeLine<RT>("x is 100, finally!"))
select x;

Contents

Prelude

when <F> (bool flag, K<F, Unit> alternative)
unless <F> (bool flag, K<F, Unit> alternative)
guard <F> (bool flag)
guard <E> (bool flag, Func<E> False)
guard <E> (bool flag, E False)
guard (bool flag, Func<Error> False)
guard (bool flag, Error False)
guardnot <E> (bool flag, Func<E> True)
guardnot <E> (bool flag, E True)
guardnot (bool flag, Func<Error> True)
guardnot (bool flag, Error True)

class Prelude Source #

Methods

method K<F, Unit> when <F> (bool flag, K<F, Unit> alternative) Source #

where F : Applicative<F>

Conditional execution of Applicative expressions

Run the alternative when the flag is true, return pure () when false

Parameters

param	flag	If `true` the `alternative` is run
param	alternative	Computation to run if the `flag` is `true`
returns	Either the result of the `alternative` computation if the `flag` is `true` or `Unit`

Examples

from x in ma
from _ in when(x == 100, Console.writeLine<RT>("x is 100, finally!"))
select x;

method K<F, Unit> unless <F> (bool flag, K<F, Unit> alternative) Source #

where F : Applicative<F>

Conditional execution of Applicative expressions

Run the alternative when the flag is false, return pure () when true

Parameters

param	flag	If `false` the `alternative` is run
param	alternative	Computation to run if the flag is `false`
returns	Either the result of the `alternative` computation if the `flag` is `false` or `Unit`

Examples

from x in ma
from _ in unless(x == 100, Console.writeLine<RT>("x should be 100!"))
select x;

method K<F, Unit> guard <F> (bool flag) Source #

where F : Alternative<F>

Guard against continuing an applicative expression

Parameters

param	flag	Flag for continuing
returns	Applicative that yields `()` if `flag` is `true`; otherwise it yields `Applicative.Empty` - shortcutting the expression

method Guard<E, Unit> guard <E> (bool flag, Func<E> False) Source #