unless

unless :: Applicative f => Bool -> f () -> f ()
base Control.Monad, relude Relude.Bool.Reexport, ghc-internal GHC.Internal.Control.Monad
The reverse of when.

Examples

>>> do x <- getLine
unless (x == "hi") (putStrLn "hi!")
comingupwithexamplesisdifficult
hi!

>>> unless (pi > exp 1) Nothing
Just ()
unless :: Applicative f => Bool -> f () -> f ()
base-compat Control.Monad.Compat, protolude Protolude.Monad, rio RIO.Prelude, base-prelude BasePrelude, turtle Turtle, shelly Shelly Shelly.Lifted Shelly.Pipe, classy-prelude ClassyPrelude, Cabal-syntax Distribution.Compat.Prelude, universum Universum.Bool.Reexport, ihaskell IHaskellPrelude, basement Basement.Compat.Base Basement.Imports, base-compat-batteries Control.Monad.Compat, Agda Agda.Utils.Monad, rebase Rebase.Prelude, hledger Hledger.Cli.Script, persistent-test Init, massiv-test Test.Massiv.Utils, incipit-base Incipit.Base, LambdaHack Game.LambdaHack.Core.Prelude, cabal-install-solver Distribution.Solver.Compat.Prelude, faktory Faktory.Prelude, can-i-haz Control.Monad.Except.CoHas Control.Monad.Reader.Has, vcr Imports, verset Verset, yesod-paginator Yesod.Paginator.Prelude, control-monad-free Control.Monad.Free, distribution-opensuse OpenSuse.Prelude, hledger-web Hledger.Web.Import, termonad Termonad.Prelude
The reverse of when.
unless :: (Boolean bool, ToBool bool, Monad m) => bool -> m () -> m ()
cond Control.Conditional
Generalization of unless
unless :: (Monoid e) => (a -> Bool) -> Wire s e m a a
netwire Control.Wire.Interval
When the given predicate is false for the input signal.
  • Depends: now.
  • Inhibits: unless the predicate is false.
unless :: (Monad m k, PreArrow k, u ~ UnitObject k, ObjectPair k (m u) u) => Bool -> m u `k` m u
constrained-categories Control.Category.Constrained.Prelude Control.Monad.Constrained
unless :: Monoid m => Bool -> m -> m
BNFC BNFC.Utils
Generalization of unless.
unless :: () => Bool -> Cont r () -> Cont r ()
control-dsl Control.Dsl Control.Dsl.Cont
type family Unless (p :: Bool) (err :: Constraint) :: Constraint
optics-core Optics.Internal.Optic.TypeLevel
Show a custom type error if p is false (or stuck).
type family Unless (a :: Bool) (a1 :: f ()) :: f ()
singletons-base Control.Monad.Singletons
Unless :: Condition -> Effect -> Effect
LambdaHack Game.LambdaHack.Content.ItemKind
if condition met, fail without a message; better avoided, since AI can't value it well
unlessXOptM :: Extension -> TcRnIf gbl lcl () -> TcRnIf gbl lcl ()
ghc GHC.Tc.Utils.Monad
unlessM :: Monad m => m Bool -> m () -> m ()
ghc GHC.Utils.Monad
Monadic version of unless, taking the condition in the monad
unlessM :: Monad m => m Bool -> m () -> m ()
extra Control.Monad.Extra Extra
Like unless, but where the test can be monadic.
unlessLeft :: Applicative m => Either a b -> (b -> m ()) -> m ()
either Data.Either.Combinators
A synonym of whenRight.
unlessRight :: Applicative m => Either a b -> (a -> m ()) -> m ()
either Data.Either.Combinators
A synonym of whenLeft.
unlessExpert :: FormatM () -> FormatM ()
hspec-core Test.Hspec.Core.Formatters.V2
Do nothing on `--expert`, otherwise run the given action.
unlessM :: Monad m => m Bool -> m () -> m ()
protolude Protolude.Bool
unlessM :: Monad m => m Bool -> m () -> m ()
relude Relude.Bool.Guard
Monadic version of unless. Reverse of whenM. Conditionally don't execute the provided action. 
>>> unlessM (pure False) $ putTextLn "No text :("
No text :(

>>> unlessM (pure True) $ putTextLn "Yes text :)"
unlessM :: Monad m => m Bool -> m () -> m ()
rio RIO.Prelude
Run the second value if the first value returns False
unlessM :: Monad m => m Bool -> m () -> m ()
shelly Shelly Shelly.Lifted Shelly.Pipe
A monadic-conditional version of the unless guard.
unlessM :: Monad m => m Bool -> m () -> m ()
classy-prelude ClassyPrelude
Only perform the action if the predicate returns False. Since 0.9.2
unlessF :: Bool -> Builder -> Builder
fmt Fmt Fmt.Internal.Formatters
Display something only if the condition is False (empty string otherwise).
unlessLeft :: Monad m => Either a b -> (b -> m ()) -> m ()
either-unwrap Data.Either.Unwrap
A synonym of whenRight.
unlessRight :: Monad m => Either a b -> (a -> m ()) -> m ()
either-unwrap Data.Either.Unwrap
A synonym of whenLeft.
unlessM :: Monad m => m Bool -> m () -> m ()
universum Universum.Bool.Guard
Monadic version of unless. 
>>> unlessM (pure False) $ putTextLn "No text :("
No text :(

>>> unlessM (pure True) $ putTextLn "Yes text :)"