[atrettel]

The article did not discuss this, but to me, one of the bigger differences between Fortran and more modern languages is the difference between functions and subroutines. Yes, they are not synonyms in Fortran and serve different purposes. I think this would trip up more people initially than the clunky syntax.

It is also a bit funny that the author complains about older Fortran programs requiring SCREAMING_CASE, when if anything this is an improvement over previous and current practices. Too many Fortran codes have overly terse variable names that often were just single characters or impenetrable abbreviations for obscure terms. I have had to create cheat sheets for each program to figure out what each variable was.

Sun Microsystems had a great quote about this back in the day [1]:

> Consistently separating words by spaces became a general custom about the tenth century A.D., and lasted until about 1957, when FORTRAN 77 abandoned the practice.

[1] https://docs.oracle.com/cd/E19957-01/802-2998/802-2998.pdf

[Luminary099]

Huh, I remember actually being taught this at school, but they never bothered to give (or I never bothered to remember) an example of a programming language that actually named void functions differently or indeed why it couldn't just be a void function. Looking at it now, it seems to be a difference inherited from mathematics, which would also explain why it's in Fortran too.

[adrian_b]

The main difference between functions and subroutines in Fortran and other ancient programming languages is not the fact that subroutines do not have a return value.

The functions of Fortran are what would be called pure functions in C (which can be marked as such with compilers that support C language extensions, like gcc).

The pure functions cannot modify any of their arguments or any global variable, and they must be idempotent, which is important in program optimization.

[pklausler]

This is complete misinformation and you should stop posting it.

One can explicitly declare a function (or subroutine) to be PURE in Fortran, but it is not the default and never has been.

[pklausler]

Spaces don't matter in fixed-form Fortran source files, so I don't get that at all. And case doesn't matter in either source form.

[atrettel]

Yes, sorry for the confusion. To be clear, the quote is directly about spaces not being significant in the source code in general, but I was commenting more about how this mindset affects variable names in practice. At least in my experience, many codes would benefit from variables names that use underscores.

[IAmBroom]

What practical difference ever existed, beyond the fact that a subroutine does not return a value? AFAIK variable scope was handled identically. Recursion was likewise identical (forbidden originally).

[adrian_b]

The very important difference is that what are called functions in Fortran are called pure functions in other languages, i.e. functions that do not modify their arguments or global variables and which are idempotent, helping program optimization.

This means that Fortran functions are functions in the mathematical sense. In most early programming languages "functions" were functions in the mathematical sense, while other kinds of subprograms were named procedures or subroutines. The use of the term "function" for any kind of procedure has been popularized mainly by the C language, even if there were earlier languages where all procedures could return a value and even where any program statement was an expression, starting with LISP.

Many C/C++ compilers, e.g. gcc, support language extensions allowing functions to be marked as pure. This is always recommended where applicable, for better program optimization.

This difference is much more important than the fact that subroutines do not return a value.

Many of the "functions" of C/C++ must be written as subroutines in Fortran, with an extra argument for the result, but because they modify some arguments or global variables, not because they were written as "void" functions in C/C++.

[semi-extrinsic]

Functions return a value, subroutines do not. So functions can, at the whim of the compiler, cause an extra copy.

Style wise, many prefer to reserve functions for things that resemble mathematical functions (i.e. only intent(in) and pure). In some sense a little bit similar to how people tend to use lambdas in python.

[adrian_b]

In a well designed programming language, the compiler should always decide at its whim, whether input or output parameters need an extra copy or not, i.e. if they should be passed by value or by reference.

The programmer must only specify the behavior of the parameters, i.e. if they are input, output or input-output parameters, like in Ada.

The fact that a parameter is the result is just a matter of syntax, not of semantics. Any other output parameters should behave exactly like the result. This means that for any output parameter, like also for the result, the compiler must decide between receiving the output value in a register or passing an extra pointer on input that is the address of a memory area where the function must write the output value.

[semi-extrinsic]

Functions, the way we use them in mathematical equations, have a particular syntax. The point of functions in Fortran is to mimic this as closely as reasonably possible (consider it is a language with a long history).

Giving the user low-level control of how memory is used can be very useful for writing fast code. The compiler is not omniscient. Providing the choice is not bad language design.

[atrettel]

They are pretty similar, but they are definitely used differently. For one, you have to "call" a subroutine in one statement, but you can use multiple functions on the same statement (since they can return values). Functions (usually) do not change their arguments, but subroutines often do. In some sense, functions are closer to how mathematical functions work but subroutines are closer to labels for certain procedures.

[adastra22]

So they are used differently, but there isn’t a language enforced difference (other than return value)?

[adrian_b]

There is a language enforced difference.

Fortran functions correspond to "pure" functions in C/C++ and other languages, i.e. idempotent functions that do not modify arguments or global variables.

If a C/C++ function is converted to Fortran, it must be rewritten as a subroutine, unless it is a pure function.

Not all C/C++ compilers support the language extension of marking functions as pure, and even with such compilers many programmers are lazy, so they forget to mark as pure the functions where this is applicable, even if this is recommended for improving program optimization and verification.

Fortran function were pure functions because this is the mathematical sense of the term "function". The extension of the meaning of the word "function" for any kind of procedure happened decades after the creation of Fortran.

The distinction between a "void" function and other functions has negligible importance. On the other hand the distinction between "functions" in the C language sense and "pure functions" is very important and programmers should better mark as "pure" all the functions for which this is true. This is at least as important for program optimization and for checking program correctness as declaring a variable with the correct type.

[pklausler]

> Fortran functions correspond to "pure" functions in C/C++ and other languages, i.e. idempotent functions that do not modify arguments or global variables.

This is nonsense. Fortran functions aren't pure. They can have side effects.

HPF/Fortran '95 added the PURE attribute for subprograms, but it's not the default.

[pklausler]

Functions are called only within the context of expression evaluation, and Fortran allows a compiler to perform algebraic transformations on expressions. If you write X=Y*F(Z) and we can determine that Y is zero, the function call can be deleted. So side effects in functions are somewhat risky.

[atrettel]

The language enforced difference is that only functions can return a value, but other than that, they are quite similar and just called "procedures" generally. In my experience, Fortran programmers use them differently in practice, and that is more of a guideline than something enforced by the language itself.

[whynotmaybe]

> difference between functions and subroutines.

Waitaminit, is that why we have "sub" in Visual Basic ?

[dhosek]

Pascal also distinguishes between functions and subroutines (procedures)

[thatjoeoverthr]

Yes! QBasic also, IIRC.

[adzm]

it was a great improvement over GOSUB... which i probably have not thought about in forever

[auraham]

I was about to mention that.

[jolt42]

I find it annoying in Java when uppercase is used because its an acronym. URL or UUID. No man, just Url/Uuid. No need to yell just because its an acronym.