epicours/Algo/Séminaire/Chapter 3 - Case analysis.md

3.1. The alternative

The if structure

if cond then expr1 else expr2

⚠️ expr1 and expr2 have to be the same type. cond is a bool

For exemple

# if 1<2 then "higher" else "lower" ;;
-: string = "higher"

Exercise : absolute program

# let abs(x) = 
	if x>0 then x
	else -x ;;;
val abs : int -> int <fun>

3.2. Exceptions

Division by 0:

# 1/0
Exception : Division by zero

Failwith:

Failwith is a function that take a string argument (the reason) and return the error. That way, we can raise an error into our code

# failwith;;
-: string -> a = <fun>

# failwith "oops";;
Exception : Failwith "oops"

invalid_arg:

Invalid_arg is a function that take a string argument (the reason) and return the error. That way, we can raison an error about an argument

# invalid_arg "oops";
Exception: Invalid_argument "oops"

(*example*)
# let div a b = 
	if b = 0 then invalid_arg("div: b = 0")
	else a/b ;;;
val div int -> int -> int = <fun>

3.3. Filtering

Explicit filtering

Explicit means that you matches the value

General syntax

match expr with pattern1 -> result1 | pattern2 -> result2 | ... | patternn -> resultn 

All expressions and pattern must avec the same type

Example

# let f x = match x with 
	0 -> 18
	| 1 -> 24
	| y ->  y * y;;

val f : int -> int = <fun>

# f 1;;
- : int = 24

# let g x = match x with
	0 -> 18
	| y -> y + y (*here will return a warning : unused case*)
	| 1 -> 24 ;;

val f : int -> int = <fun>
# f 1;;
- : int = 1

# let d x = match x with 
	0 -> 18;
	| 1 -> 24;;
	(* Warning = pattern matching not exhaustive *)

Filtering several values

# let and_ a b =
	match a with 
		true -> (match b with 
			true -> true
			| false -> false)
		| false -> false;;

⚠️ If you forgot the parentheses, Caml will belived the two |false belong to the same pattern matching

Universal pattern

# let f x = match x with 
	0 -> 18
	| 1 -> 24
	| _ -> x*x ;;

"Or" filter

# let f x match x with 
	0 | 20 -> 18
	| 1 | 11 | 12 -> 24
	|_ -> x * x ;;

Guarded filters

# let sign x = match x with 
	0 -> 0
	| y when y < 0 -> -1
	| _ -> 1 

y when y<0 is an evaluation