Haskell Language Typed holes Semantics of typed holes


The value of a type hole can simply said to be undefined, although a typed hole triggers a compile-time error, so it is not strictly necessary to assign it a value. However, a typed hole (when they are enabled) produces a compile time error (or warning with deferred type errors) which states the name of the typed hole, its inferred most general type, and the types of any local bindings. For example:

Prelude> \x -> _var + length (drop 1 x)

<interactive>:19:7: Warning:
    Found hole `_var' with type: Int
    Relevant bindings include
      x :: [a] (bound at <interactive>:19:2)
      it :: [a] -> Int (bound at <interactive>:19:1)
    In the first argument of `(+)', namely `_var'
    In the expression: _var + length (drop 1 x)
    In the expression: \ x -> _var + length (drop 1 x)

Note that in the case of typed holes in expressions entered into the GHCi repl (as above), the type of the expression entered also reported, as it (here of type [a] -> Int).