doc: added section on coproducts and documented their pattern matching in TUTORIAL.md

1 files changed, 38 insertions, 1 deletions

diff --git a/TUTORIAL.md b/TUTORIAL.md
index 0f6a21a..9c85478 100644
--- a/TUTORIAL.md
+++ b/TUTORIAL.md
@@ -222,6 +222,21 @@ calculate the sum of a vector of integers. Through
 type conversions, this works for lists too.
 
 
+Coproducts
+----------
+
+Coproducts ("either-or -type") are instantiated with the
+`coprod` operator:
+```console
+> (coprod (+ 1 2) Bool)
+(coprod 3 Bool) : (Sum Int Bool)
+> (coprod Int +)
+(coprod Int +) : (Sum Int ((Int ...) -> Int))
+```
+They can be destructured with pattern matching as covered in
+the next section.
+
+
 Pattern matching
 ----------------
 
@@ -272,6 +287,25 @@ myvec saved
 1 : Int
 ```
 
+Coproducts are destructured with `inl` and `inr`:
+```console
+> (case myprod ((inl x) x) ((inr b) (if b 1 0)) (_ -1))
+2 : Int
+> (case myprod ((inl 2) true) (_ false))
+true : Bool
+```
+Even if when both an `inl` and `inr` arm are supplied,
+a wildcard pattern is required as myslip doesn't feature an
+exhaustiveness checker. That is the tradeoff for having
+nesting:
+```console
+> (case (quote myprod 1) (((inl 2) _) true) (_ false))
+true : Bool
+```
+(though with a careful rewrite of type checking for pattern
+matching, at least exhaustiveness for inl/inr arms would be
+fairly trivial, but that won't happen this summer)
+
 
 General recursion
 -----------------
@@ -296,6 +330,7 @@ The factorial function could be implemented as such:
 ))
 ```
 
+
 Printing
 --------
 
@@ -323,10 +358,12 @@ These exercises are meant to make you familiar with the
 language. The solutions can be found in the `solutions`
 folder under the project root.
 
+
 Exercise 1: Fibonacci sequence
 ------------------------------
 
-Write a function that calculates the n:th fibonacci number.
+Write a function that calculates the n:th fibonacci number,
+when n is given as the sole argument.
 
 This should make you familiar with function definitions and
 fixed point recursion.