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82 lines
2.1 KiB
Markdown
82 lines
2.1 KiB
Markdown
# Iterative
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π-calculus has a notion of the repetitive process: `!P = P | !P`. That means, you can always fork a new `P` process if you need it.
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In Aqua, two operations correspond to it: you can call a service function (it's just available when it's needed), and you can use `for` loop to iterate on collections.
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### `for` expression
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In short, `for` looks like the following:
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```haskell
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xs: []string
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for x <- xs:
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y <- foo(x)
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-- x and y are not accessible there, you can even redefine them
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x <- bar()
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y <- baz()
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```
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## Contract
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* Iterations of `for` loop are executed sequentially by default.
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* Variables defined inside `for` loop are not available outside.
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* `for` loop's code has access to all variables above.
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* `for` can be executed on a variable of any [Collection type](../types.md#collection-types).
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### Conditional `for`
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For can be executed on a variable of any [Collection type](../types.md#collection-types). You can make several trials in a loop, and break once any trial succeeded.
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```haskell
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xs: []string
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for x <- xs try:
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-- Will stop trying once foo succeeds
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foo(x)
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```
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The contract is changed as in [Parallel](parallel.md#contract) flow.
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### Parallel `for`
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Running many operations in parallel is the most commonly used pattern for `for`.
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```
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xs: []string
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for x <- xs par:
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on x:
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foo()
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-- Once the fastest x succeeds, execution continues
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-- If you want to make the subsequent execution independent from for,
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-- mark it with par, e.g.:
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par continueWithBaz()
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```
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The contract is changed as in [Conditional](conditional.md#contract) flow.
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### Export data from `for`
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The way to export data from `for` is the same as in [Conditional return](conditional.md#conditional-return) and [Race patterns](parallel.md#join-behavior).
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```haskell
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xs: []string
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return: *string
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-- can be par, try, or nothing
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for x <- xs par:
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on x:
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return <- foo()
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-- Wait for 6 fastest results -- see Join behavior
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baz(return!5, return)
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```
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### `for` on streams
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`for` on streams is one of the most advanced and powerful parts of Aqua. See [CRDT streams](../crdt-streams.md) for details.
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