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Source file src/go/types/operand.go

Documentation: go/types

     1  // Copyright 2012 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // This file defines operands and associated operations.
     6  
     7  package types
     8  
     9  import (
    10  	"bytes"
    11  	"fmt"
    12  	"go/ast"
    13  	"go/constant"
    14  	"go/token"
    15  )
    16  
    17  // An operandMode specifies the (addressing) mode of an operand.
    18  type operandMode byte
    19  
    20  const (
    21  	invalid   operandMode = iota // operand is invalid
    22  	novalue                      // operand represents no value (result of a function call w/o result)
    23  	builtin                      // operand is a built-in function
    24  	typexpr                      // operand is a type
    25  	constant_                    // operand is a constant; the operand's typ is a Basic type
    26  	variable                     // operand is an addressable variable
    27  	mapindex                     // operand is a map index expression (acts like a variable on lhs, commaok on rhs of an assignment)
    28  	value                        // operand is a computed value
    29  	commaok                      // like value, but operand may be used in a comma,ok expression
    30  	commaerr                     // like commaok, but second value is error, not boolean
    31  	cgofunc                      // operand is a cgo function
    32  )
    33  
    34  var operandModeString = [...]string{
    35  	invalid:   "invalid operand",
    36  	novalue:   "no value",
    37  	builtin:   "built-in",
    38  	typexpr:   "type",
    39  	constant_: "constant",
    40  	variable:  "variable",
    41  	mapindex:  "map index expression",
    42  	value:     "value",
    43  	commaok:   "comma, ok expression",
    44  	commaerr:  "comma, error expression",
    45  	cgofunc:   "cgo function",
    46  }
    47  
    48  // An operand represents an intermediate value during type checking.
    49  // Operands have an (addressing) mode, the expression evaluating to
    50  // the operand, the operand's type, a value for constants, and an id
    51  // for built-in functions.
    52  // The zero value of operand is a ready to use invalid operand.
    53  //
    54  type operand struct {
    55  	mode operandMode
    56  	expr ast.Expr
    57  	typ  Type
    58  	val  constant.Value
    59  	id   builtinId
    60  }
    61  
    62  // Pos returns the position of the expression corresponding to x.
    63  // If x is invalid the position is token.NoPos.
    64  //
    65  func (x *operand) Pos() token.Pos {
    66  	// x.expr may not be set if x is invalid
    67  	if x.expr == nil {
    68  		return token.NoPos
    69  	}
    70  	return x.expr.Pos()
    71  }
    72  
    73  // Operand string formats
    74  // (not all "untyped" cases can appear due to the type system,
    75  // but they fall out naturally here)
    76  //
    77  // mode       format
    78  //
    79  // invalid    <expr> (               <mode>                    )
    80  // novalue    <expr> (               <mode>                    )
    81  // builtin    <expr> (               <mode>                    )
    82  // typexpr    <expr> (               <mode>                    )
    83  //
    84  // constant   <expr> (<untyped kind> <mode>                    )
    85  // constant   <expr> (               <mode>       of type <typ>)
    86  // constant   <expr> (<untyped kind> <mode> <val>              )
    87  // constant   <expr> (               <mode> <val> of type <typ>)
    88  //
    89  // variable   <expr> (<untyped kind> <mode>                    )
    90  // variable   <expr> (               <mode>       of type <typ>)
    91  //
    92  // mapindex   <expr> (<untyped kind> <mode>                    )
    93  // mapindex   <expr> (               <mode>       of type <typ>)
    94  //
    95  // value      <expr> (<untyped kind> <mode>                    )
    96  // value      <expr> (               <mode>       of type <typ>)
    97  //
    98  // commaok    <expr> (<untyped kind> <mode>                    )
    99  // commaok    <expr> (               <mode>       of type <typ>)
   100  //
   101  // commaerr   <expr> (<untyped kind> <mode>                    )
   102  // commaerr   <expr> (               <mode>       of type <typ>)
   103  //
   104  // cgofunc    <expr> (<untyped kind> <mode>                    )
   105  // cgofunc    <expr> (               <mode>       of type <typ>)
   106  //
   107  func operandString(x *operand, qf Qualifier) string {
   108  	var buf bytes.Buffer
   109  
   110  	var expr string
   111  	if x.expr != nil {
   112  		expr = ExprString(x.expr)
   113  	} else {
   114  		switch x.mode {
   115  		case builtin:
   116  			expr = predeclaredFuncs[x.id].name
   117  		case typexpr:
   118  			expr = TypeString(x.typ, qf)
   119  		case constant_:
   120  			expr = x.val.String()
   121  		}
   122  	}
   123  
   124  	// <expr> (
   125  	if expr != "" {
   126  		buf.WriteString(expr)
   127  		buf.WriteString(" (")
   128  	}
   129  
   130  	// <untyped kind>
   131  	hasType := false
   132  	switch x.mode {
   133  	case invalid, novalue, builtin, typexpr:
   134  		// no type
   135  	default:
   136  		// should have a type, but be cautious (don't crash during printing)
   137  		if x.typ != nil {
   138  			if isUntyped(x.typ) {
   139  				buf.WriteString(x.typ.(*Basic).name)
   140  				buf.WriteByte(' ')
   141  				break
   142  			}
   143  			hasType = true
   144  		}
   145  	}
   146  
   147  	// <mode>
   148  	buf.WriteString(operandModeString[x.mode])
   149  
   150  	// <val>
   151  	if x.mode == constant_ {
   152  		if s := x.val.String(); s != expr {
   153  			buf.WriteByte(' ')
   154  			buf.WriteString(s)
   155  		}
   156  	}
   157  
   158  	// <typ>
   159  	if hasType {
   160  		if x.typ != Typ[Invalid] {
   161  			buf.WriteString(" of type ")
   162  			WriteType(&buf, x.typ, qf)
   163  		} else {
   164  			buf.WriteString(" with invalid type")
   165  		}
   166  	}
   167  
   168  	// )
   169  	if expr != "" {
   170  		buf.WriteByte(')')
   171  	}
   172  
   173  	return buf.String()
   174  }
   175  
   176  func (x *operand) String() string {
   177  	return operandString(x, nil)
   178  }
   179  
   180  // setConst sets x to the untyped constant for literal lit.
   181  func (x *operand) setConst(tok token.Token, lit string) {
   182  	var kind BasicKind
   183  	switch tok {
   184  	case token.INT:
   185  		kind = UntypedInt
   186  	case token.FLOAT:
   187  		kind = UntypedFloat
   188  	case token.IMAG:
   189  		kind = UntypedComplex
   190  	case token.CHAR:
   191  		kind = UntypedRune
   192  	case token.STRING:
   193  		kind = UntypedString
   194  	default:
   195  		unreachable()
   196  	}
   197  
   198  	val := constant.MakeFromLiteral(lit, tok, 0)
   199  	if val.Kind() == constant.Unknown {
   200  		x.mode = invalid
   201  		x.typ = Typ[Invalid]
   202  		return
   203  	}
   204  	x.mode = constant_
   205  	x.typ = Typ[kind]
   206  	x.val = val
   207  }
   208  
   209  // isNil reports whether x is the nil value.
   210  func (x *operand) isNil() bool {
   211  	return x.mode == value && x.typ == Typ[UntypedNil]
   212  }
   213  
   214  // assignableTo reports whether x is assignable to a variable of type T. If the
   215  // result is false and a non-nil reason is provided, it may be set to a more
   216  // detailed explanation of the failure (result != ""). The check parameter may
   217  // be nil if assignableTo is invoked through an exported API call, i.e., when
   218  // all methods have been type-checked.
   219  func (x *operand) assignableTo(check *Checker, T Type, reason *string) (bool, errorCode) {
   220  	if x.mode == invalid || T == Typ[Invalid] {
   221  		return true, 0 // avoid spurious errors
   222  	}
   223  
   224  	V := x.typ
   225  
   226  	// x's type is identical to T
   227  	if check.identical(V, T) {
   228  		return true, 0
   229  	}
   230  
   231  	Vu := V.Underlying()
   232  	Tu := T.Underlying()
   233  
   234  	// x is an untyped value representable by a value of type T.
   235  	if isUntyped(Vu) {
   236  		if t, ok := Tu.(*Basic); ok && x.mode == constant_ {
   237  			return representableConst(x.val, check, t, nil), _IncompatibleAssign
   238  		}
   239  		return check.implicitType(x, Tu) != nil, _IncompatibleAssign
   240  	}
   241  	// Vu is typed
   242  
   243  	// x's type V and T have identical underlying types and at least one of V or
   244  	// T is not a named type.
   245  	if check.identical(Vu, Tu) && (!isNamed(V) || !isNamed(T)) {
   246  		return true, 0
   247  	}
   248  
   249  	// T is an interface type and x implements T
   250  	if Ti, ok := Tu.(*Interface); ok {
   251  		if m, wrongType := check.missingMethod(V, Ti, true); m != nil /* Implements(V, Ti) */ {
   252  			if reason != nil {
   253  				if wrongType != nil {
   254  					if check.identical(m.typ, wrongType.typ) {
   255  						*reason = fmt.Sprintf("missing method %s (%s has pointer receiver)", m.name, m.name)
   256  					} else {
   257  						*reason = fmt.Sprintf("wrong type for method %s (have %s, want %s)", m.Name(), wrongType.typ, m.typ)
   258  					}
   259  
   260  				} else {
   261  					*reason = "missing method " + m.Name()
   262  				}
   263  			}
   264  			return false, _InvalidIfaceAssign
   265  		}
   266  		return true, 0
   267  	}
   268  
   269  	// x is a bidirectional channel value, T is a channel
   270  	// type, x's type V and T have identical element types,
   271  	// and at least one of V or T is not a named type
   272  	if Vc, ok := Vu.(*Chan); ok && Vc.dir == SendRecv {
   273  		if Tc, ok := Tu.(*Chan); ok && check.identical(Vc.elem, Tc.elem) {
   274  			if !isNamed(V) || !isNamed(T) {
   275  				return true, 0
   276  			} else {
   277  				return false, _InvalidChanAssign
   278  			}
   279  		}
   280  	}
   281  
   282  	return false, _IncompatibleAssign
   283  }
   284  

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