A lesson to help beginners dodge various syntax traps

ocaml-lessons/lesson5/lesson.html

@@ -1 +1 @@
-<h3>-</h3>
+<h3>Syntax Traps</h3>

ocaml-lessons/lesson5/step1/step.html

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-<h3>This lesson is not yet available</h3>
+<h3>Sequence of expressions</h3>
 
-<p>Use <code>lessons ()</code> to get the list of all lessons.
-Use <code>back ()</code> to return to the previous step.</p>
+<p>It is easy to get confused by OCaml syntax since it's different from
+mainstream languages. So you'd better be aware of a few things before digging
+any deeper.</p>
+
+<p>To begin with, you should know that a proper command must normally ends with
+<em>';;'</em> to be processed by the top-level.  This tutorial automatically adds
+the double semicolon as soon as you hit enter but the normal top-level won't.
+The double semicolon is only required when interacting with the top level
+interpreter and as such is not part of OCaml syntax.</p>
+
+<p>What <em>is</em> part of OCaml syntax, though, is the simple semicolon
+<em>';'</em> which is commonly used as an expression terminator, except that in
+OCaml it's a <b>separator</b>.  In other words, you must not write <code>expr1;
+expr2;</code> but <code>expr1 ; expr2</code>.</p>
+
+<p>You may now enter <code>next ()</code> to check your understanding.</p>

ocaml-lessons/lesson5/step1/step.ml

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 fun input output ->
-  find_in  "- : char =" output
+  false

ocaml-lessons/lesson5/step2/step.html

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+<h3>Exercise: Get the Punctuation Right!</h3>
+
+<p>Let's see if you got this right. Here is a sequence of erroneous
+commands. Your task is to fix all of them in order to get the correct
+answer at the end.</p>
+
+<p><code>let fernand = "King of Castille";</code></p>
+
+<p><code>let rodrigue = "The cid"; let diegue = "cid's father"</code></p>
+
+<p><code>characters = [ fernand;; rodrigue;; diegue ]</code></p>
+
+<p><code>rodrigue.[4] <- 'C' ; diegue.[0] <- rodrigue.[4] ;</code></p>
+
+<p><code>characters</code></p>
+

ocaml-lessons/lesson5/step2/step.ml

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+fun input output ->
+  find_in "characters" input &&
+  output = "- : string list = [\"King of Castille\"; \"The Cid\"; \"Cid's father\"]\n"

ocaml-lessons/lesson5/step3/step.html

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+<h3>The <em>let</em> keyword</h3>
+
+<p>The other source of confusion for newcomers is the <em>let</em> keyword
+which acts differently in the toplevel than in normal OCaml expressions.</p>
+
+<p>In the toplevel <code>let x = 1</code> binds the name <em>x</em> to the
+integer 1 as seen in <code>lesson 2</code>. If x was already bound to something
+it's previous binding is lost:</p>
+<code>let x = "I am now a string!"</code>
+
+<p>The <em>let</em> keyword is also used to form an expression in which a name
+is given to some value temporarily, for the evaluation of a subexpression only:
+<code>let x = 41 in x + 1</code>. The value of <em>x</em> is <em>41</em> during
+the evaluation of <em>x + 1</em> only; the global binding of <em>x</em> to
+<code>"I am now a string!"</code> is preserved.</p>
+
+<p>See what <code>x</code> is evaluated to now, and type <code>next ()</code>
+for a little practice.</p>

ocaml-lessons/lesson5/step3/step.ml

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+fun input output ->
+  false

ocaml-lessons/lesson5/step4/step.html

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+<h3>Exercise: let there be lets!</h3>
+
+<p>Fix all these <em>let</em> expressions in order to get the expected result
+at the end:</p>
+
+<pre><code>let xy =
+  let x = 'x' and let y = 'y' in x ::[y]</code></pre>
+
+<pre><code>let ab =
+  let a = 'a'
+  let b = 'B' in Char.lowercase b
+  in a ::[b]
+</code></pre>
+
+<pre><code>let up = Char.uppercase in
+  big_xy = List.map up xy ;
+  big_ab = List.map up ab ;
+  big_ab @ big_xy</code></pre>

ocaml-lessons/lesson5/step4/step.ml

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+fun input output ->
+  find_in "Char.uppercase" input &&
+  output = "- : char list = ['A'; 'B'; 'X'; 'Y']\n"

ocaml-lessons/lesson5/step5/step.html

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+<h3>Parentheses</h3>
+
+<p>With regard to grouping expression or enforcing order of evaluation,
+OCaml syntax is surprisingly easy: you can use pervasively either parentheses
+of <em>begin</em>/<em>end</em> keywords.</p>
+
+<p>Example grouping expressions in an <em>if</em> form:</p>
+<pre><code>if 1+2 = 3 then (
+  print_string "did you knew that?\n" ;
+  print_string "amazing!\n"
+)
+</code></pre>
+
+<p>Or forcing order of evaluation with <em>begin</em>/<em>end</em> (although
+you won't find this often!):</p>
+<code>begin 1 + 2 end * 3</code>
+
+<p>Also, as function application takes precedence over infix operators you will
+frequently uses parentheses to make explicit the expected evaluation order, as
+in: <code>square (1 + 1)</code> since <code>square 1+1</code> would yield
+<em>2</em>.
+
+<p>Enter <code>next ()</code> when you are ready to practice.</p>

ocaml-lessons/lesson5/step5/step.ml

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+fun input output ->
+  false

ocaml-lessons/lesson5/step6/step.html

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+<h3>Exercise: Fix the grouping</h3>
+
+<p>A Lisp programmer stole all our parentheses!
+Get them back in order to get the proper result at the end.</p>
+
+<pre><code>let ten =
+  let double x = x+x in
+  double 3 + 2</code></pre>
+
+<pre><code>let hundred =
+  if true or false then
+    print_string "May I help you?\n" ;
+    100
+  else 0</code></pre>
+
+<pre><code>let one =
+  let accum = ref -54 in
+  for i = 1 to ten do accum := !accum + i done ;
+  !accum </code></pre>
+
+<pre><code>one + match hundred with
+  | 42  -> match ten with 10 -> 52  | _ -> 0
+  | 100 -> match ten with 10 -> 110 | _ -> 0
+</code></pre>
+

ocaml-lessons/lesson5/step6/step.ml

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+fun input output ->
+  find_in "match" input && output = "- : int = 111\n"