_________________________

                        ÜBUNGSSTUNDE 2013-10-31

                            Luminous Fennell
                       _________________________


1 Errata
========

  - threepenny-gui (`cabal install ... threepenny-gui')
  - bmp Installation (Mac OS X)
  - import von `Ex01' *und* `CalcMain' `:l Ex01.hs CalcMain.hs'
  - `dup' nicht erklärt `dup [1,2,3,...] == [1,1,2,3,...]'
  - `calcMain' füttert Wörter an `readCommand', (statt Zeilen)

  Korrekturen werden in die Installationsanleitung aufgenommen!


2 Modules and Imports
=====================

  - Declaration
    ,----
    | -- This is the top of the file
    | module Ex01_Solution where
    | -- ...
    `----
  - Import:
    ,----
    | import Ex01_Solution
    `----

  - Modules correspond to filenames
  - Modules have capital names
  - If no module is declared, the default is `Main'
  - The module `Prelude' is imported by default


3 Name Clashes
==============

  Loading the following `.hs'-File:
  ,----
  | curry :: String
  | curry = "yes"
  | 
  | travelDestination :: String
  | travelDestination = if curry == "yes"
  |                     then "India"
  |                     else "France"
  `----
  Yields a compilation error:
  ,----
  | Ambiguous occurrence `curry'
  |  It could refer to either `Main.curry',
  |                           defined at LabExamples_scratch.hs:3:1
  |                        or `Prelude.curry',
  |                           imported from `Prelude' at LabExamples_scratch.hs:1:1
  |                           (and originally defined in `Data.Tuple')
  `----


4 Resolving Name Clashes: Selective import
==========================================

  - By default, importing a module includes all the funtions it defines.
  - Using a special import form lets us choose the imported functions
    explicitly:
  ,----
  | import Prelude (String, (==))
  | -- Here, we only import the two listed functions
  | --   that we need from Prelude
  | 
  | curry :: String
  | curry = "yes"
  | 
  | travelDestination :: String
  | travelDestination = if curry == "yes" then "India" else "France"
  `----
  but we have to list everything we need from Prelude...


5 Resolving Name Clashes: hiding
================================

  Alternatively, we can prevent certain functions from being imported:
  ,----
  | import Prelude hiding (curry)
  | 
  | curry :: String
  | curry = "yes"
  | -- ...
  `----


6 Qualified imports
===================

  - Sometimes it may be necessary to use both, our definitions, and that
    of the module.
  - *Qualified imports* allow to specify a custom prefix for imported
    functions.
  ,----
  | import qualified Prelude as P
  | -- qualified import
  | 
  | curry :: P.String
  | -- String needs to be prefixed with P to be recognized
  | curry = "yes"
  | 
  | -- this is the curry from Prelude
  | defaultCurry = P.curry
  | 
  | -- (==) also needs to be prefixed
  | travelDestination :: P.String
  | travelDestination = if curry P.== "yes" then "India" else "France"
  `----


7 Qualified imports
===================

  In order to avoid prefixing everything, we can combine hiding and
  qualified imports
  ,----
  | import Prelude hiding (curry)
  | import qualified Prelude as P
  | 
  | 
  | -- only `curry' is hidden, no need to prefix `String'
  | curry :: String
  | curry = "yes"
  | 
  | -- this is the curry from Prelude
  | defaultCurry = P.curry
  | 
  | travelDestination :: String
  | travelDestination = if curry == "yes" then "India" else "France"
  `----