====== First Haskell programs ======

  * main function with output, as a complete code example
    * compiles, error and warning free, with compiler: GHC 8.10.4, using compiler option -Wall
    * <code Haskell>
main :: IO ()
main = print 'x'
</code>

  * executes with the following output:
    * <code>'x'
</code>

  * concise explanation of the code
    * function ''main'' is always the entry point of a Haskell program
      * this is the function that demands the evaluation of the function right to the equal sign (''='')
    * function ''main'' equals to function ''print'' with '''x' '' as the first parameter
    * NOTE: The function ''print'' is a function that will only be evaluated when its evaluation is demanded. Hence, the function ''print'' not a command.
  * concise explanation of the code elements
    * {{:codesnippets:firstprogramwithfunctionmain.png|}}
    * first line declares that function ''main'' has type ''IO ()''
    * second line declares that function ''main'' equals to ''print 'x' ''
      * whereas ''print'', is a function that is imported from a default package
      * and 'x' is the character ''x''
    * default package is ''[[codesnippets:prelude|Prelude]]''
    * in the second line ''main'', ''print'', and '''x' '' are functions
      * ''main'' has no parameter but has type ''IO ()''
      * ''print'' has one parameter and has type ''Show a => a -> IO ()''
      * '''x''' has no parameter, and has type ''Char''
    * ''IO'' is a parametric type declared as ''data IO a''
      * whereas a is a type parameter, that can be any type
    * ''()'' is the [[https://en.wikipedia.org/wiki/Unit_type|unit type]]
    * hence ''IO ()'' is the data type ''IO'' with type parameter ''()''


===== Explanation, step by step =====

  * Haskell code repeated, from above
    * <code Haskell>
main :: IO ()
main = print 'x'
</code>

==== first line declares that function ''main'' has type ''IO ()'' ====

  * ''IO ()'' is just a type signature with the resulting type ''IO ()'', and no parameters
  * see the following code as examples for different type signatures:
    * <code Haskell>
main :: IO ()
main = printString sResult

printString :: String -> IO ()
printString s = print s

sResult :: String
sResult = fun 5 'a'

fun :: Int -> Char -> String
fun n ch = replicate n ch
</code>
  * {{:codesnippets:typesignatureoffunctionfun.png?nolink|}}
  * symbol ''->'' separates parameter from other parameter, and separates from the result type that is on the right hand side
  * the example shows the follwing 4 type signatures
    * ''IO ()''
      * result type: ''IO ()'', no parameter
    * ''String -> IO ()''
      * result type: ''IO ()'', type of parameter: ''String''
    * ''String''
      * result type: ''String''
    * ''Int -> Char -> String''
      * result type: ''String'', type of first parameter: ''Int'', type of second parameter: ''String''
  * NOTE:
    * Types start always with upper case letters.
    * Type signatures can also have type parameters, and type constraints.
    * Type type parameters start always with lower case letters.
  * the example executes with the following output:
    * <code>
"aaaaa"
</code>

==== second line declares that function ''main'' equals to ''print 'x''' ====

  * expression '' main = print 'x' '' is called a [[codesnippets:bindingandpatternmatching|binding]]
    * it binds ''print 'x' '' to function ''main''
    * when function ''main'' evaluates it evaluates ''print'' with parameter '''x' ''
  * expression ''sResult = fun 5 'a' '' is just another [[codesnippets:bindingandpatternmatching|binding]]
    * it binds ''fun 5 'a' '' to function ''sResult''
    * when function ''sResult'' evaluates it evaluates ''fun'' with parameter ''5'' and '''a' ''

==== default package is Prelude ====

  * ''[[codesnippets:prelude|Prelude]]'' is part of library ''base'' ([[https://hackage.haskell.org/package/base-4.15.0.0/docs/Prelude.html|Prelude]])
  * package ''[[codesnippets:prelude|Prelude]]'' includes standard modules (e.g. module System.IO)
  * enables basic elements like functions, types, type classes, and instances
  * the import of such elements can be controlled in different ways
    * see also [[codesnippets:avoidconflictprelude|Avoiding name conflict with Prelude]]


===== ✎ =====
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