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Often when I'm playing with Haskell code, I stub things out with a type annotation and undefined.
foo :: String -> Int
foo = undefined
Is there a type-level "undefined" that I could use in a similar way?
(Ideally, in conjunction with a kind annotation)
type Foo :: * -> *
type Foo = Undefined
Further thought on the same thread: is there a way for me to stub out typeclass instances for types created this way? An even easier way than the following theoretical way?
instance Monad Foo where
return = undefined
(>>=) = undefined
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A variable that has not been assigned a value is of type undefined . A method or statement also returns undefined if the variable that is being evaluated does not have an assigned value. A function returns undefined if a value was not returned .
The undefined datatype, whose value is undefined is used to denote an absence of a meaningful value. A variable in JavaScript that is without any value has a value of undefined. The datatype of a variable that holds an undefined value is also 'undefined'.
undefined does not equal undefined.
You can assign a avariable with undefined. testVar = undefined; //typeof(testVar) will be equal to undefined. Save this answer.
You can use EmptyDataDecls to stub out a type, and with KindSignatures you can give it a kind:
{-# LANGUAGE EmptyDataDecls, KindSignatures #-}
data Foo :: * -> *
You can also stub out the Monad instance without warnings with this option to GHC.
{-# OPTIONS_GHC -fno-warn-missing-methods #-}
instance Monad Foo
And then you don't need to leave any implementation for return and >>=.
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This question was asked and answered a long time ago; best practices have evolved since.
These days, instead of undefined, for stubbing out code you should be using typed holes, and their type-level analogue, partial type signatures.
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