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|---|---|---|
 | Chapter 5. Patterns |  |
| 5.7. Cons Pattern | 5.8. As Pattern | 5.9. Wildcard Pattern |
| ||
|---|---|---|
| | Chapter 5. Patterns | |
| 5.7. Cons Pattern | 5.8. As Pattern | 5.9. Wildcard Pattern |
The as keyword binds a variable to the value matched by a pattern.
It is a more general way of binding variables than using variable patterns alone.
When P as x is matched
against a type T, the pattern P
is matched against T, producing typing context Γ.
The typing context produced by the whole match is Γ ∪ {x has type T}.
{- Consider this initial program fragment, without an 'as' pattern -} val (a,b) = ((1,2),(3,4)) val (ax,ay) = a val (bx,by) = b {- Compared to the following fragment -} val ((ax,ay) as a, (bx,by) as b) = ((1,2),(3,4)) [ax, ay, a] | [bx, by, b] {- OUTPUT:PERMUTABLE [1, 2, (1, 2)] [3, 4, (3, 4)] -}