Lesson 27 · String concatenation - Build a Programming Language

String concatenation

Strings become useful when you can join them. Today you evaluate + on two strings, producing a new string object with their contents concatenated.

The goal

Evaluate the + operator on two string operands into a concatenated string object.

Start here - the target

TO DO

Scenario: Concatenating strings with plus

Giventhe source "foo" + "bar"

Whenthe evaluator evaluates the expression

Thenthe result is a string object whose value is foobar

Andevaluating "" + "x" yields the string x

Background

You parsed string literals back in the parsing chapter, but the evaluator has not had a String object until now. Define it - a wrapper over the text with the usual Type/Inspect - and teach the infix evaluator one new case: when both operands are strings and the operator is +, produce a new string with their contents joined.

Only + is defined on strings; -, *, and comparisons will be type errors handled in the next lesson but one. Keep the boundary in mind - "" + "x" is "x", an empty string concatenates cleanly - and resist adding more string operations today. Concatenation alone makes strings first-class enough to build messages and, later, to see printed output from your programs.

Make it work

type String struct { Value string }
func (s *String) Type() string { return "STRING" }
// in the infix evaluator, when both operands are strings and op is "+":
//   return &String{ left.Value + right.Value }
// other operators on strings will be a type error later

CheckpointDONE

The + operator concatenates two strings into a new string object. Commit.