Finding & assembling class dictionaries from GHC plugins?

Simon Peyton-Jones simonpj at
Tue Nov 5 09:13:34 UTC 2013

I'm not sure what you mean by "synthesise".

Typeable instances can no longer be hand-written; they must be derived by GHC.  That's to keep them type-secure.

A core-level plug-in can't generate fresh instances.  There are some new plug-in hooks that fit earlier in the pipeline, which can.  Maybe you can use that, and generate a data type decl with "deriving Typeable"?


From: ghc-devs [mailto:ghc-devs-bounces at] On Behalf Of Conal Elliott
Sent: 04 November 2013 20:08
To: ghc-devs at
Subject: Finding & assembling class dictionaries from GHC plugins?

I'm working on a GHC plugin (for compiling Haskell to hardware), and I need to synthesize Typeable (and maybe other) class dictionaries for a wide range of types, including composite types (functions, pairs, lists, etc). Can it be done, and how? I think I'm mainly looking for mechanics of finding existing class instances (however they're defined) and assembling them (for parametrized/composite dictionaries), rather than mechanisms specific to Typeable.
Thanks, -- Conal
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