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Source file src/go/types/eval.go

Documentation: go/types

     1  // Copyright 2013 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  package types
     6  
     7  import (
     8  	"fmt"
     9  	"go/parser"
    10  	"go/token"
    11  )
    12  
    13  // Eval returns the type and, if constant, the value for the
    14  // expression expr, evaluated at position pos of package pkg,
    15  // which must have been derived from type-checking an AST with
    16  // complete position information relative to the provided file
    17  // set.
    18  //
    19  // If pkg == nil, the Universe scope is used and the provided
    20  // position pos is ignored. If pkg != nil, and pos is invalid,
    21  // the package scope is used. Otherwise, pos must belong to the
    22  // package.
    23  //
    24  // An error is returned if pos is not within the package or
    25  // if the node cannot be evaluated.
    26  //
    27  // Note: Eval should not be used instead of running Check to compute
    28  // types and values, but in addition to Check. Eval will re-evaluate
    29  // its argument each time, and it also does not know about the context
    30  // in which an expression is used (e.g., an assignment). Thus, top-
    31  // level untyped constants will return an untyped type rather then the
    32  // respective context-specific type.
    33  //
    34  func Eval(fset *token.FileSet, pkg *Package, pos token.Pos, expr string) (_ TypeAndValue, err error) {
    35  	// determine scope
    36  	var scope *Scope
    37  	if pkg == nil {
    38  		scope = Universe
    39  		pos = token.NoPos
    40  	} else if !pos.IsValid() {
    41  		scope = pkg.scope
    42  	} else {
    43  		// The package scope extent (position information) may be
    44  		// incorrect (files spread across a wide range of fset
    45  		// positions) - ignore it and just consider its children
    46  		// (file scopes).
    47  		for _, fscope := range pkg.scope.children {
    48  			if scope = fscope.Innermost(pos); scope != nil {
    49  				break
    50  			}
    51  		}
    52  		if scope == nil || debug {
    53  			s := scope
    54  			for s != nil && s != pkg.scope {
    55  				s = s.parent
    56  			}
    57  			// s == nil || s == pkg.scope
    58  			if s == nil {
    59  				return TypeAndValue{}, fmt.Errorf("no position %s found in package %s", fset.Position(pos), pkg.name)
    60  			}
    61  		}
    62  	}
    63  
    64  	// parse expressions
    65  	node, err := parser.ParseExprFrom(fset, "eval", expr, 0)
    66  	if err != nil {
    67  		return TypeAndValue{}, err
    68  	}
    69  
    70  	// initialize checker
    71  	check := NewChecker(nil, fset, pkg, nil)
    72  	check.scope = scope
    73  	check.pos = pos
    74  	defer check.handleBailout(&err)
    75  
    76  	// evaluate node
    77  	var x operand
    78  	check.rawExpr(&x, node, nil)
    79  	check.processDelayed(0) // incl. all functions
    80  
    81  	return TypeAndValue{x.mode, x.typ, x.val}, nil
    82  }
    83  

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