github.com/xyproto/u-root@v6.0.1-0.20200302025726-5528e0c77a3c+incompatible/cmds/core/elvish/eval/vals/concat.go (about)

     1  package vals
     2  
     3  import (
     4  	"errors"
     5  	"fmt"
     6  )
     7  
     8  // Concatter wraps the Concat method. See Concat for how it is used.
     9  type Concatter interface {
    10  	// Concat concatenates the receiver with another value, the receiver being
    11  	// the left operand. If concatenation is not supported for the given value,
    12  	// the method can return the special error type ErrCatNotImplemented.
    13  	Concat(v interface{}) (interface{}, error)
    14  }
    15  
    16  // RConcatter wraps the RConcat method. See Concat for how it is used.
    17  type RConcatter interface {
    18  	RConcat(v interface{}) (interface{}, error)
    19  }
    20  
    21  // ErrConcatNotImplemented is a special error value used to signal that
    22  // concatenation is not implemented. See Concat for how it is used.
    23  var ErrConcatNotImplemented = errors.New("cat not implemented")
    24  
    25  // Concat concatenates two values. If both operands are strings, it returns lhs
    26  // + rhs, nil. If the left operand implements Concatter, it calls
    27  // lhs.Concat(rhs). If lhs doesn't implement the interface or returned
    28  // ErrConcatNotImplemented, it then calls rhs.RConcat(lhs). If all attempts
    29  // fail, it returns nil and an error.
    30  func Concat(lhs, rhs interface{}) (interface{}, error) {
    31  	if v, ok := tryConcatBuiltins(lhs, rhs); ok {
    32  		return v, nil
    33  	}
    34  
    35  	if lhs, ok := lhs.(Concatter); ok {
    36  		v, err := lhs.Concat(rhs)
    37  		if err != ErrConcatNotImplemented {
    38  			return v, err
    39  		}
    40  	}
    41  
    42  	if rhs, ok := rhs.(RConcatter); ok {
    43  		v, err := rhs.RConcat(lhs)
    44  		if err != ErrConcatNotImplemented {
    45  			return v, err
    46  		}
    47  	}
    48  
    49  	return nil, fmt.Errorf("unsupported concat: %s and %s", Kind(lhs), Kind(rhs))
    50  }
    51  
    52  func tryConcatBuiltins(lhs, rhs interface{}) (interface{}, bool) {
    53  	switch lhs := lhs.(type) {
    54  	case string:
    55  		if rhs, ok := rhs.(string); ok {
    56  			return lhs + rhs, true
    57  		}
    58  	}
    59  
    60  	return nil, false
    61  }