- @goto label
- @label label
Overuse or inappropriate use of advanced control flow makes code hard to read.
@goto or its equivalents in other languages, when used improperly, leads to unreadable spaghetti code.
Similar to languages like C, one cannot jump between functions in Julia. This also means that
@goto is not possible at the top-level; it will only work within a function. Furthermore, one cannot jump from an inner function to its outer function, or from an outer function to an inner function.
In languages such as C, the
@goto statement is often used to ensure a function cleans up necessary resources, even in the event of an error. This is less important in Julia, because exceptions and
finally blocks are often used instead.
However, it is possible for Julia code to interface with C code and C APIs, and so sometimes functions still need to be written like C code. The below example is contrived, but demonstrates a common use case. The Julia code will call
Libc.malloc to allocate some memory (this simulates a C API call). If not all allocations succeed, then the function should free the resources obtained so far; otherwise, the allocated memory is returned.
using Base.Libc function allocate_some_memory() mem1 = malloc(100) mem1 == C_NULL && @goto fail mem2 = malloc(200) mem2 == C_NULL && @goto fail mem3 = malloc(300) mem3 == C_NULL && @goto fail return mem1, mem2, mem3 @label fail free(mem1) free(mem2) free(mem3) end
Although not traditionally considered loops, the
@label macros can be used for more advanced control flow. One use case is when the failure of one part should lead to the retry of an entire function, often useful in input validation:
function getsequence() local a, b @label start print("Input an integer: ") try a = parse(Int, readline()) catch println("Sorry, that's not an integer.") @goto start end print("Input a decimal: ") try b = parse(Float64, readline()) catch println("Sorry, that doesn't look numeric.") @goto start end a, b end
However, this use case is often more clear using recursion:
function getsequence() local a, b print("Input an integer: ") try a = parse(Int, readline()) catch println("Sorry, that's not an integer.") return getsequence() end print("Input a decimal: ") try b = parse(Float64, readline()) catch println("Sorry, that doesn't look numeric.") return getsequence() end a, b end
Although both examples do the same thing, the second is easier to understand. However, the first one is more performant (because it avoids the recursive call). In most cases, the cost of the call does not matter; but in limited situations, the first form is acceptable.