UndefinedTypeVariable Error

Example

id :: a -> a
id x = x

Cause

This error occurs when an undefined type variable appears in a type signature.

In the example, the type variable a is undefined.

Note that PureScript requires all type variables to be defined in type signatures, unlike in Haskell. See also Differences from Haskell.

Fix

• Check the spelling of any type variables.

• A possible fix is to introduce the variable using a forall quantifier:

  purescriptCopy

  id :: forall a. a -> a
  id x = x
  

Notes