Case Expressions

Example (newtype is defined in a previous post).

fun f (x : newtype) =
  case x of
    Pizza => 3
  | Str s => 8
  | TwoInts(i1, i2) => i1 + i2

To call f:

f(TwoInts(3,4))

In a case expression, the expressions of all branches must have the same type.

General syntax:

case e0 of
      p1 => e1
    | p2 => e2
    | p3 => e3

The pi are called patterns. They are not expressions. We do not evaluate them.

All the types of ei must be the same.

The compiler checks:

• No missing cases (warning)
• No redundant cases (error)

You can use case anywhere you use an expression. It is often the whole function body but need not be.

Evaluation:

Evaluate e0 to v0. Then the first pattern that matches v0 is used. The environment is augmented with ei. Note that if the pattern is a constructor C(x1,...,xn), then each xi is bound to vi in the environment of ei.

fun inc_or_zero int option =
  case intoption of
      NONE => 0
    | SOME i => i+1