By Christian Findlay 25 Jul 2026

Block Expressions

A block evaluates local bindings and expressions in order, then yields its trailing expression. Default delimits a block with braces; ML uses an indented layout block. Both lower to Expr::Block { statements, value }.

block ::= "{" statement* expression? "}"

let result = {
    let x = 10
    let y = 20
    x + y
}

result =
    x = 10
    y = 20
    x + y

Evaluation and scope [BLOCK-SCOPE]

• Statements run from top to bottom.
• The trailing expression can use bindings introduced earlier in the block.
• Each block has a child lexical scope. An inner binding may shadow an outer binding; leaving the inner block restores the outer binding.
• A binding introduced in a block is not visible after that block ends.
• An outer binding remains readable inside nested blocks.

let x = 100
let result = {
    let x = 50
    let inner = {
        let x = 25
        x
    }
    x + inner
}
// result is 75; the outer x remains 100

Result value

A block with a trailing expression has that expression's type and value. A block without one yields Unit.