Go gotcha

26 pitfalls and corner cases

We hope this collection of common mistakes in Go will help you spot similar traps and save you time when debugging your own code.

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Table of contents

1. Go gotcha: Why can't I add elements to my map?
   You can't assign an entry to a nil map in Go.
2. Go gotcha: What's a nil pointer dereference?
   You can't follow the nil pointer.
3. Go gotcha: Multiple values in single value context?
   If a function returns multiple values, you must use all of them.
4. Go gotcha: Why doesn't this function change my array?
   Arrays in Go are values: when you pass an array to a function it gets a copy of the original array data.
5. Go gotcha: Two variables with the same name?
   An identifier declared in a block may be redeclared in an inner block.
6. Go gotcha: Extra comma in slice literal
   In a multi-line slice, array or map literal, every line must end with a comma.
7. Go gotcha: Why can't I update my string?
   Go strings are read-only byte slices (with a few extra properties).
8. Go gotcha: Why aren't the characters concatenated?
   Characters (rune literals) are numbers in Go.
9. Go gotcha: What happened to ABBA?
   The Trim functions strip all Unicode code points contained in a cutset.
10. Go gotcha: What happened to my copy?
    Copy copies the minimum number of elements in the destination and source slices.
11. Go gotcha: Why doesn't append work every time?
    If there is enough capacity, append reuses the underlying array.
12. Go gotcha: Why can't I print large numbers? (constant overflows int)
    An untyped integer constant is converted to an int when the type can't be inferred from the context.
13. Go gotcha: Why doesn't increment (++) and decrement (--) work?
    In Go increment and decrement are statements written with postfix notation.
14. Go gotcha: Why is my computation wrong?
    The multiplication, division, and remainder operators have the same precedence.
15. Go gotcha: Why does Go and Pythagoras disagree?
    The circumflex (^) denotes bitwise XOR in Go.
16. Go gotcha: Why doesn't this loop end?
    An integer overflow occurs when an arithmetic operation tries to create a value that is outside the range that can be represented.
17. Go gotcha: Numbers that start with zero
    Octal literals start with 0, hexadecimal with 0x, an decimal literals never start with zero.
18. Go gotcha: What's wrong with the remainder (modulo) operator?
    The remainder operator can give negative answers if the dividend is negative.
19. Go gotcha: Why can't I multiply a time.Duration with an integer?
    There is no mixing of numeric types in Go.
20. Go gotcha: Why is this index out of range?
    Arrays, slices and strings are indexed starting from zero.
21. Go gotcha: Unexpected values in range loop
    The range loop generates two values: first the index, then the data.
22. Go gotcha: Can't change entries in range loop
    The range loop uses a local variable to store iteration values.
23. Go gotcha: Iteration variable doesn't see change in range loop
    The range expression is evaluated once before beginning the loop.
24. Go gotcha: Iteration variables and closures
    A data race occurs when two goroutines access the same variable concurrently and at least one of the accesses is a write.
25. Go gotcha: Why is the JSON output empty?
    Only the the exported fields of a Go struct will be present in the JSON output.
26. Go gotcha: Why is nil not equal to nil?
    An interface value is nil only if the concrete value and dynamic type are both nil.