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70 lines
1.7 KiB
Go
70 lines
1.7 KiB
Go
package merge
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import (
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"fmt"
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"errors"
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)
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// Boxed is a boxed value that is able to merge with other values. Boxing a
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// value pushes its type-checking from compile time to run time. This can be
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// useful in the event that you wish to construct mergeable values with
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// heterogeneous members, but should likely be avoided otherwise.
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type Boxed struct {
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val interface{}
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ident func() interface{}
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merge func(interface{}) error
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}
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func (b Boxed) Merge(from Boxed) error {
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if err := b.merge(from.val); err != nil {
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return fmt.Errorf("boxed merge failed: %w", err)
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}
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return nil
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}
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func (b Boxed) MergeIdentity() Boxed {
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return Boxed{
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val: b.ident(),
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ident: b.ident,
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merge: b.merge,
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}
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}
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var typeMismatch = errors.New("mismatched types")
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// strip takes a typed function and erases the type information from its
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// parameter
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func strip[X any](f func(X) error) func(interface{}) error {
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return func(v interface{}) error {
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vv, ok := v.(X)
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if !ok {
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return fmt.Errorf("unexpected %T value, expected %T instead: %w", v, vv, typeMismatch)
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}
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return f(vv)
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}
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}
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func stripReturn[X any](f func() X) func() interface{} {
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return func() interface {} { return f() }
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}
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// Box takes a mergeable value and creates a new mergeable value of type Boxed.
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// Any two Boxed values can attempt to merge at runtime.
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func Box[X Merges[X]](x X) Boxed {
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return Boxed{
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val: x,
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ident: stripReturn(x.MergeIdentity),
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merge: strip(x.Merge),
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}
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}
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// Unbox removes a value from its box.
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func Unbox[X Merges[X]](b Boxed) (X, error) {
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if v, ok := b.val.(X); ok {
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return v, nil
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} else {
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var zero X
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return zero, fmt.Errorf("box contains %T, not %T: %w", b.val, zero, typeMismatch)
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}
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}
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