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== Working environment ==

Using the installation guide from [[Functional programming | Functional programming]] prepare your working environment.

===How to verify it is working?===

*Run <code>ghci</code> from the command line. it will start GHC Interpreter, the output should be something like this:
<syntaxhighlight lang="Haskell" class="myDark" >
GHCi, version 8.6.5: http://www.haskell.org/ghc/ :? for help
Prelude>
</syntaxhighlight >
*If you write expression, for example <code>1+2*3</code>, it should be evaluated. You can close this interpreter by typing <code>:q</code>.
*Open Visual Studio Code, create a file <code>simple.hs</code> and put there following lines of code:
<syntaxhighlight lang="Haskell" >
doubleMe x = x * x

plus x y = x + y + 'a'
</syntaxhighlight >
It should report a problem in function <code>plus</code> (you can not use <code>+</code> with letter, it can be repaired by removing <code>+ 'a'</code>). Report from VS Code is refreshed whenever the source file is saved on disk.

== First program in Haskell ==
In your favorite development environment (VS Code by default):
* Crate a file <code>simple.hs</code>.
* Create a function <code>pythagoras a b</code> that computes <code>c</code> based on <math>c^2 = a^2 + b^2 \,,</math>. Necessary functions can be found: [https://hoogle.haskell.org/ Hoogle]
* Open <code>ghci</code> and run the implemented function with <code>3 4</code>. File can be loaded using command <code>:l</code> (<code>:load</code>) and reloaded with <code>:r</code> (<code>:reload</code>).

== Real first program in Haskell ==

If you are complaining, that all programming courses should start with printing <code>"Hello world!"</code> on the screen and that is why previous task sucks. Do the following exercise.
*Crate a file <code>firstProgram.hs</code> with following content:
<syntaxhighlight lang="Haskell">
main = do putStr "Hello world!"
</syntaxhighlight >
*Compile it with 'GHC compiler' (command <code>ghc</code>).
*It should produce an executable file <code>firstProgram(.exe)</code>, run this file.

@@ Line 28: / Line 28: @@
 pythagoras a b = sqrt (a*a + b*b)
 </syntaxhighlight>
 </div>
 <div style="clear:both"></div>

@@ Line 19: / Line 19: @@
 It should report a problem in function <code>plus</code> (you can not use <code>+</code> with letter, it can be repaired by removing <code>+ 'a'</code>). Report from VS Code is refreshed whenever the source file is saved on disk.
 == First program in Haskell ==
 In your favorite development environment (VS Code by default):
 * Crate a file <code>simple.hs</code>.
 * Create a function <code>pythagoras a b</code> that computes <code>c</code> based on  <math>c^2 = a^2 + b^2 \,,</math>. Necessary functions can be found: [https://hoogle.haskell.org/ Hoogle]
 * Open <code>ghci</code> and run the implemented function with <code>3 4</code>. File can be loaded using command <code>:l</code> (<code>:load</code>) and reloaded with <code>:r</code> (<code>:reload</code>).
+<div class="mw-collapsible mw-collapsed" data-collapsetext="Hide solution" data-expandtext="Show solution">
 == Real first program in Haskell ==