Question

I am trying to implement map function in VB.NET and I have tried the below.

 Function Map(a,f)
    Dim i
    for each i in a
      f(i)
    next
 End Function

 Function alert(a)
    MessageBox.Show(a)
 end function

But the above code is not working and saying alert is not declared.
Please help me on this.

Answer 1

Your functions don’t return anything. Try this:

Public Sub Map(Of T)(ByVal a As IEnumerable(Of T), ByVal f As Action(T))
    For Each i As T In a
      f(i)
    Next
End Sub

Public Sub alert(ByVal a As Object)
    MessageBox.Show(a)
End Sub

Above is based on the methods from the question. An actual traditional Map() function might look more like this:

Public Iterator Function Map(Of T)(ByVal a As IEnumerable(Of T), ByVal f As Func(Of T,T)) As IEnumerable(Of T)
    For Each i As T In a
        Yield f(i)
    Next
End Function

Or this:

Public Iterator Function Map(Of T, U)(ByVal a As IEnumerable(Of T), ByVal f As Func(Of T, U)) As IEnumerable(Of U)
    For Each i As T In a
        Yield f(i)
    Next
End Function