Which makes a lot of GHC code more readable — I’m serious! Manuel
PS: I have resisted it for a while, but after slogging through GHC for extended periods, I’ve come to appreciate the additional clarity in large and tricky functions (e.g., in the type checker & renamer). Edward Kmett <ekm...@gmail.com>: > That has a high chance of backfiring and requiring everyone to use do { ...; > ... } with explicit braces and semis. ;) > > -Edward > > > On Wed, Jul 2, 2014 at 4:08 AM, Simon Marlow <marlo...@gmail.com> wrote: > Agreed, let's do it. Thanks for the well-argued proposal. > > Next up: consistent style :-) > > Cheers, > Simon > > > On 27/06/2014 10:51, Johan Tibell wrote: > Hi! > > I found myself exploring new parts of the GHC code base the last few > weeks (exciting!), which again reminded me of my biggest frustration > when working on GHC: the lack of per-function/type (Haddock) comments. > > GHC code is sometimes commented with "notes", which are great but tend > to (1) mostly cover the exceptional cases and (2) talk about the > implementation of a function, not how a caller might use it or why. > > Lack of documentation, in GHC and other software projects, usually has > (at least) two causes: > > * Programmers comment code they think is "complex enough to warrant a > > comment". The problem is that the author is usually a poor judge of > what's complex enough, because he/she is too familiar with the code > and tends to under-document code when following this principle. > * Documenting is boring and tends to have little benefit the person > > writing to documentation. Given lack of incentives we tend to > document less than we ought to. > > I've only seen one successful way to combat the lack of documentation > that stems from the above: have the project's style guide mandate that > top-level functions and types (or at least those that are exported) have > documentation. This works well at Google. > > Anecdote: we have one code base inside Google that was until recently > exempt from this rule and documentation is almost completely absent in > that code base, even though hundreds of engineers work on and need to > understand it every day. This breeds institutional knowledge problems > i.e. if the author of a core piece of code leaves, lots of knowledge is > lost. > > *Proposal: *I propose that we require that new top-level functions and > > types have Haddock comments, even if they start out as a single, humble > sentence. > > I've found that putting even that one sentence (1) helps new users and > (2) establishes a place for improvements to be made. There's a strong > "broken window" effect to lack of comments, in that lack of comments > breeds more lack of comments as developers follow established practices. > > We should add this requirement to the style guide. Having it as a > written down policy tends to prevent having to re-hash the whole > argument about documentation over and over again. This has also helped > us a lot at Google, because programmers can spend endless amount of time > arguing about comments, placement of curly braces, etc. and having a > written policy helps cut down on that. > > To give an idea of how to write good comments, here are two examples of > undocumented code I ran into in GHC and how better comments would have > helped. > > *First example* > > In compiler/nativeGen/X86/Instr.hs there's a (local) function called > mkRUR, which is a helper function use when computing instruction > register usage. > > The first question that I asked upon seeing uses of that function was > "what does RUR stand for?" Given the context the function is in, I > guessed it stands for read-update-read, because R is used to mean "read" > in the enclosing function and "updating" is related to "reading" so that > must be what U stands for. It turns out that it stands for > RegUsageReadonly. Here's a comment that would have captured, in a single > sentence, what this function is for: > > -- | Create register usage info for instruction that only > -- reads registers. > mkRUR src = src' `seq` RU src' [] > where src' = filter (interesting platform) src > > That already a big improvement. A note about the register filtering, > which means that not all registers you pass to the function will be > recorded as being read in the end, could also be useful. > > Aside: providing a type signature, which would have made it clear that > the return type is RU, might also have helped in this particular case. > > *Second example* > > In the same file there a function called x86_regUsageOfInstr. It's the > function that encloses the local function mkRUR above. > > I could figure out that this function has something to do with register > usage, of the instruction passed as an argument, and that register usage > is important for the register allocator. However, trying to understand > in more detail what that meant was more of challenge than it needed to > be. First, a comment more clearly explaining what computing register > usage means in practice would be helpful: > > -- | Returns which registers are read and written by this > -- instruction, as a (read, written) pair. This info is used > -- by the register allocator. > x86_regUsageOfInstr :: Platform -> Instr -> RegUsage > > The reason mentioning that the return value is essentially a (read, > written) pair is helpful is because the body of the function a big case > statement full of lines like this one: > > GCMP _ src1 src2 -> mkRUR [src1,src2] > ... > FDIV _ src dst -> usageRM src dst > > It's not immediately clear that all the various helper functions used > here just end up computing a pair of the above form. A top-level comment > lets you understand what's going on without understanding exactly what > all these helper functions are doing. > > Thoughts? > > -- Johan > > > > _______________________________________________ > ghc-devs mailing list > ghc-devs@haskell.org > http://www.haskell.org/mailman/listinfo/ghc-devs > > _______________________________________________ > ghc-devs mailing list > ghc-devs@haskell.org > http://www.haskell.org/mailman/listinfo/ghc-devs > > _______________________________________________ > ghc-devs mailing list > ghc-devs@haskell.org > http://www.haskell.org/mailman/listinfo/ghc-devs
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