f :: a -> a f x = _

hole.hs:2:7: Found hole `_' with type: a Where: `a' is a rigid type variable bound by the type signature for f :: a -> a at hole.hs:1:6 In the expression: _ In an equation for `f': f x = _ Relevant bindings include x :: a (bound at hole.hs:2:3) f :: a -> a (bound at hole.hs:2:1) Valid hole fits include x :: a (bound at hole.hs:2:3)

cons z = z : True : _x : y

Foo.hs:3:21: error: Found hole: _x :: Bool Or perhaps ‘_x’ is mis-spelled, or not in scope In the first argument of ‘(:)’, namely ‘_x’ In the second argument of ‘(:)’, namely ‘_x : y’ In the second argument of ‘(:)’, namely ‘True : _x : y’ Relevant bindings include z :: Bool (bound at Foo.hs:3:6) cons :: Bool -> [Bool] (bound at Foo.hs:3:1) Valid hole fits include z :: Bool (bound at mpt.hs:2:6) otherwise :: Bool (imported from ‘Prelude’ at mpt.hs:1:8-10 (and originally defined in ‘GHC.Base’)) False :: Bool (imported from ‘Prelude’ at mpt.hs:1:8-10 (and originally defined in ‘GHC.Types’)) True :: Bool (imported from ‘Prelude’ at mpt.hs:1:8-10 (and originally defined in ‘GHC.Types’)) maxBound :: forall a. Bounded a => a with maxBound @Bool (imported from ‘Prelude’ at mpt.hs:1:8-10 (and originally defined in ‘GHC.Enum’)) minBound :: forall a. Bounded a => a with minBound @Bool (imported from ‘Prelude’ at mpt.hs:1:8-10 (and originally defined in ‘GHC.Enum’)) Foo.hs:3:26: error: Variable not in scope: y :: [Bool]

cons = _x : _x

unbound.hs:1:8: Found hole '_x' with type: a Where: `a' is a rigid type variable bound by the inferred type of cons :: [a] at unbound.hs:1:1 In the first argument of `(:)', namely `_x' In the expression: _x : _x In an equation for `cons': cons = _x : _x Relevant bindings include cons :: [a] (bound at unbound.hs:1:1) unbound.hs:1:13: Found hole: _x :: [a] Where: ‘a’ is a rigid type variable bound by the inferred type of cons :: [a] at unbound.hs:3:1-12 Or perhaps ‘_x’ is mis-spelled, or not in scope In the second argument of ‘(:)’, namely ‘_x’ In the expression: _x : _x In an equation for ‘cons’: cons = _x : _x Relevant bindings include cons :: [a] (bound at unbound.hs:3:1) Valid hole fits include cons :: forall a. [a] with cons @a (defined at mpt.hs:3:1) mempty :: forall a. Monoid a => a with mempty @[a] (imported from ‘Prelude’ at mpt.hs:1:8-10 (and originally defined in ‘GHC.Base’))

f _x = _x + 1

import Data.List (inits) g :: [String] g = _ "hello, world"

• Found hole: _ :: [Char] -> [String] • In the expression: _ In the expression: _ "hello, world" In an equation for ‘g’: g = _ "hello, world" • Relevant bindings include g :: [String] (bound at mpt.hs:6:1) Valid hole fits include lines :: String -> [String] (imported from ‘Prelude’ at mpt.hs:3:8-9 (and originally defined in ‘base-4.11.0.0:Data.OldList’)) words :: String -> [String] (imported from ‘Prelude’ at mpt.hs:3:8-9 (and originally defined in ‘base-4.11.0.0:Data.OldList’)) inits :: forall a. [a] -> [[a]] with inits @Char (imported from ‘Data.List’ at mpt.hs:4:19-23 (and originally defined in ‘base-4.11.0.0:Data.OldList’)) repeat :: forall a. a -> [a] with repeat @String (imported from ‘Prelude’ at mpt.hs:3:8-9 (and originally defined in ‘GHC.List’)) fail :: forall (m :: * -> *). Monad m => forall a. String -> m a with fail @[] @String (imported from ‘Prelude’ at mpt.hs:3:8-9 (and originally defined in ‘GHC.Base’)) return :: forall (m :: * -> *). Monad m => forall a. a -> m a with return @[] @String (imported from ‘Prelude’ at mpt.hs:3:8-9 (and originally defined in ‘GHC.Base’)) pure :: forall (f :: * -> *). Applicative f => forall a. a -> f a with pure @[] @String (imported from ‘Prelude’ at mpt.hs:3:8-9 (and originally defined in ‘GHC.Base’)) read :: forall a. Read a => String -> a with read @[String] (imported from ‘Prelude’ at mpt.hs:3:8-9 (and originally defined in ‘Text.Read’)) mempty :: forall a. Monoid a => a with mempty @([Char] -> [String]) (imported from ‘Prelude’ at mpt.hs:3:8-9 (and originally defined in ‘GHC.Base’))

f :: Eq a => a -> Bool f x = _

show_constraints.hs:4:7: error: • Found hole: _ :: Bool • In the expression: _ In an equation for ‘f’: f x = _ • Relevant bindings include x :: a (bound at show_constraints.hs:4:3) f :: a -> Bool (bound at show_constraints.hs:4:1) Constraints include Eq a (from show_constraints.hs:3:1-22) Valid hole fits include otherwise :: Bool False :: Bool True :: Bool maxBound :: forall a. Bounded a => a with maxBound @Bool minBound :: forall a. Bounded a => a with minBound @Bool

f :: [Integer] -> Integer f = _

Valid hole fits include f :: [Integer] -> Integer head :: forall a. [a] -> a with head @Integer last :: forall a. [a] -> a with last @Integer maximum :: forall (t :: * -> *). Foldable t => forall a. Ord a => t a -> a with maximum @[] @Integer minimum :: forall (t :: * -> *). Foldable t => forall a. Ord a => t a -> a with minimum @[] @Integer product :: forall (t :: * -> *. Foldable t => forall a. Num a => t a -> a with product @[] @Integer sum :: forall (t :: * -> *). Foldable t => forall a. Num a => t a -> a with sum @[] @Integer

Valid refinement hole fits include foldl1 (_ :: Integer -> Integer -> Integer) with foldl1 @[] @Integer where foldl1 :: forall (t :: * -> *). Foldable t => forall a. (a -> a -> a) -> t a -> a foldr1 (_ :: Integer -> Integer -> Integer) with foldr1 @[] @Integer where foldr1 :: forall (t :: * -> *). Foldable t => forall a. (a -> a -> a) -> t a -> a const (_ :: Integer) with const @Integer @[Integer] where const :: forall a b. a -> b -> a ($) (_ :: [Integer] -> Integer) with ($) @'GHC.Types.LiftedRep @[Integer] @Integer where ($) :: forall a b. (a -> b) -> a -> b fail (_ :: String) with fail @((->) [Integer]) @Integer where fail :: forall (m :: * -> *). Monad m => forall a. String -> m a return (_ :: Integer) with return @((->) [Integer]) @Integer where return :: forall (m :: * -> *). Monad m => forall a. a -> m a (Some refinement hole fits suppressed; use -fmax-refinement-hole-fits=N or -fno-max-refinement-hole-fits)