Conditional logic chooses between alternatives, but statement-oriented if/then code can leave cases uncovered, assign the wrong variable, or require unsafe defaults. Expression-oriented branching makes the result explicit. section 4.1.2 section 4.1.4
Listing 4.3 Working in an expression-oriented language
let describeAge age =
let ageDescription = #1
if age < 18 then "Child" #2
elif age < 65 then "Adult"
else "OAP"
let greeting = "Hello"
$"{greeting}! You are a '{ageDescription}'."
An F# if/then expression returns a value, so each branch supplies the same result type and can initialize a binding directly. section 4.1.4
Deriving a description
let describeAge age = let ageDescription = if age < 18 then "Child" elif age < 65 then "Adult" else "OAP" $"Hello! You are a '{ageDescription}'." section 4.1.4
Choose the conditional form that matches the operation:
- Use if/then when directly testing a Boolean; it is shorter and easier to read than matching true and false. section 8.2.5
- Use if/else for a unit-returning side effect when no action is needed in the false case; the catch-all else () can be omitted. section 8.2.5
- Use pattern matching when conditional logic needs to inspect broader value shapes or cases. section 8.2.5 section 8.0
Because F# treats virtually everything as an expression, conditional branches participate in value construction, immutability, function results, and domain-modeling code rather than merely controlling program flow. section 4.1.4 section 8.4
