Pattern matching desugaring regression? Re: [Haskell-cafe] Why does my module take so long to compile?

15 Feb 2021

      Ccing ghc devs since that’s a better forum perhaps
Crazy theory:

this is a regression due the the partial changes to pattern matching
coverage checking in 8.10 that finished / landed in ghc 9

Why:
Desugaring is when pattern/ case statement translation happens I think?
And the only obvious “big thing” is that you have some huge , albeit sane
for a compiler, pattern matching

I’d first check if the new ghc 9 release doesn’t have that regression in
build time that you experienced.  And if it does file a ticket.

I may be totally wrong, but that seems like a decent likelihood !

On Mon, Feb 15, 2021 at 12:20 PM Troels Henriksen  wrote:
...
Carter Schonwald  writes:
...
How big are your data types in the file In question? Do you ghc generics
or
deriving or template Haskell? Could you share a link to the file in
question ?
The file does not define any large data types itself, but it operates on
some fairly large data types (an AST, it's a compiler).  But so do many
other modules that work just fine.  It uses no generics, nontrivial
deriving, or Template Haskell.  It's this file:
https://github.com/diku-dk/futhark/blob/master/src/Futhark/Pass/ExtractKerne...
I also found a handful of other modules in my project that are
significantly slower to compile in GHC 8.10, and seemingly also because
of the desugarer, but none of them have any obvious smoking guns like
generics or TH.
The only commonality I can find is that the affected modules contain
functions with a relatively large typeclass context.  I use
ConstraintKinds to make them more concise, and I guess unfolded there
may be 30-40 equality/class constraints in total.  Like this:
type DistLore lore =
      ( Bindable lore,
        HasSegOp lore,
        BinderOps lore,
        LetDec lore ~ Type,
        ExpDec lore ~ (),
        BodyDec lore ~ ()
      )
where the 'Bindable' constraint in particular then implies a further
handful of "~" constraints:
class
      ( ASTLore lore,
        FParamInfo lore ~ DeclType,
        LParamInfo lore ~ Type,
        RetType lore ~ DeclExtType,
        BranchType lore ~ ExtType,
        SetType (LetDec lore)
      ) =>
      Bindable lore
      where
      ...
FParamInfo/LParamInfo/etc are type families.  Are such constraints
particularly slow?
--
\  Troels
/\ Henriksen