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