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add example of using list/dict with per-element type check #471

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22 changes: 22 additions & 0 deletions docs/basics/pattern.md
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Expand Up @@ -241,6 +241,28 @@ q)((a;b):`F):3 4e
^
```

Combining type check with a list or dictionary pattern allows element-wise type checking:
```q
q)(a:`i;b:`j):(1i;2j)
q)(a:`i;b:`j):(1i;2h)
'type
[0] (a:`i;b:`j):(1i;2h)
^
q)([a:a:`i;b:b:`j]):([a:1i;b:2j])
q)([a:a:`i;b:b:`j]):([a:1i;b:2h])
'type
[0] ([a:a:`i;b:b:`j]):([a:1i;b:2h])
^
```
Note that the form ``([a:`i])`` is a dictionary pattern that matches the value at the key `a` to a symbol, so it doesn't contain a type check pattern. Additionally, using a null pattern in a type check for a dictionary element requires parentheses:
```q
q)([a:(:`i);b:(:`j)]):([a:1i;b:2j])
q)([a:(:`i);b:(:`j)]):([a:1i;b:2h])
'type
[0] ([a:(:`i);b:(:`j)]):([a:1i;b:2h])
^
```

### Filter function

The filter function pattern takes the form ```p:expr``` where `expr` is an expression that returns a callable (such as a lambda, projection or operator). The result of `expr` is called on the value from the assigned value, and the result is matched to `p`.
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