Constraint solver iterations

Simon Peyton Jones simonpj at
Thu Apr 27 14:25:22 UTC 2017

I'd love to see the repo case.

with -ddump-simpl-iterations -ddump-simpl-stats you'll see the result after each iteration, and what was done.  I bet you get to the point where only one thing happens per iteration and that's bad.

Make a ticket!



|  -----Original Message-----
|  From: ghc-devs [mailto:ghc-devs-bounces at] On Behalf Of
|  amindfv at
|  Sent: 27 April 2017 16:18
|  To: ghc-devs at
|  Subject: Constraint solver iterations
|  To build the 'vivid' package (on Hackage) with recent versions of GHC
|  (I believe the problem is with 8.0+, didn't see it before then), I
|  need to compile with -fconstraint-solver-iterations=0. (0 meaning
|  unbounded)
|  However, I noticed today that the GHC docs say "Typically one
|  iteration suffices; so please yell if you find you need to set it
|  higher than the default."
|  The code (in Vivid.SynthDef.FromUA) uses a large amount of type family
|  recursion. Is this not a case where I should be yelling? If I should
|  be, I can provide a smaller repro case.
|  If this is expected, are there techniques for rewriting the code in a
|  way that'll fit under the default number of iterations? I've tried
|  rewriting, making the number of iterations logarithmic instead of
|  linear (e.g. by appending lists instead of cons-ing), but still hit
|  the problem (even with an increased but bounded number of iteration
|  steps).
|  Thanks!
|  Tom
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