16 If Expressions

If expressions allow one expression to conditionally be evaluated instead of another expression. They use the keywords if then else.

if "batman" > "superman" then "yay" else "boo";;

- : string = "boo"

There are a few things at play here. The syntax is

if <guard> then <expression> else <expression>.

The guard must be a boolean, and both expressions must have the same type

if 0 then "yay" else "boo";;

File "[2]", line 1, characters 3-4:
1 | if 0 then "yay" else "boo";;
       ^
Error: This expression has type int but an expression was expected of type
         bool
       because it is in the condition of an if-statement

if true then "yay" else 1;;

File "[3]", line 1, characters 24-25:
1 | if true then "yay" else 1;;
                            ^
Error: This expression has type int but an expression was expected of type
         string

These both fail because of incorrect types

The if expression is very similar to the ternary operator (? :) in other languages.

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Now let's look at if expressions on paper.

Syntax

if e1 then e2 else e3

Evaluation

• if e1 ==> true and e2 ==> v, then if e1 then e2 else e3 ==> v
• if e1 ==> false and e3 ==> v, then if e1 then e2 else e3 ==> v

Type checking

if e1 : bool and e2 : t and e3 : t, then if e1 then e2 else e3 : t

It's worth noting that "==>" means "evaluates to" and ":" means "has type."