docs/programming/core/twig/cast.md · urbit.org

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5# Type manipulation (`^` runes).

7`$cast`, `$like`, and `$name` let us adjust spans without

8violating type constraints.

10The `nest` algorithm which tests span subtyping is conservative;

11it never allows invalid nests, it sometimes rejects valid nests.

13## Regular forms

15### `:like`, `^+`, "ketlus", `{$like p/twig q/twig}`

17Syntax: *2-fixed*.

19The product is the formula of `q` with the span of `p`, provided

20nouns within the span of `q` can be shown to nest within the span

21of `p`.

23### `:cast`, `^-`, "ketlus", `{$like p/moss q/twig}`

25Syntax: *2-fixed*.

27Expands to: `:like($.p q)`.

29`$like` is a typecheck by example; `$cast` is a typecheck by

30mold.

32### `:name`, `^=`, "kettis", `{$name p/@tas q/twig}

34Syntax: *2-fixed*.

36The product: the formula of `q`, with the span of `q` labeled by

37symbol `p`.

39### Irregular forms

41### Backtick cast: `\`foo\`bar`

43Translation:

44```

45`foo`bar ^-(foo bar)

46```

48### Infix name: `foo=bar`

50Translation:

51```

52foo=bar ^=(foo bar)

53```