id,summary,reporter,owner,description,type,status,priority,milestone,component,version,resolution,keywords,cc,os,architecture,failure,difficulty,testcase,blockedby,blocking,related
5138,TypeSynonymInstances should be enabled by default,mcandre,,"Source: https://github.com/mcandre/genetics

With {{{ {-# LANGUAGE TypeSynonymInstances #-} }}}, my genetic algorithm code compiles and runs perfectly.

{{{
$ make
$ ./helloga
...
hellowobld
helloworyd
helloporld
hellyworld
helloworrd
hellowofld
hellpworld
Best candidate: helloworld
}}}

Without it, Haskell refuses to declare an instance on Strings.

{{{
$ make
ghc --make -O2 -fforce-recomp helloga.hs
[1 of 2] Compiling Genetics         ( Genetics.hs, Genetics.o )
[2 of 2] Compiling Main             ( helloga.hs, helloga.o )

helloga.hs:27:10:
    Illegal instance declaration for `Gene String'
      (All instance types must be of the form (T t1 ... tn)
       where T is not a synonym.
       Use -XTypeSynonymInstances if you want to disable this.)
    In the instance declaration for `Gene String'
make: *** [all] Error 1
}}}

The fix is easy to discover and apply, but this is my first typeclass, nothing complicated. As a newbie I prefer that it ""just works"" without my having to use the special compile flag.",feature request,closed,normal,,Compiler,7.0.2,wontfix,type synonyms instance declaration,,Unknown/Multiple,Unknown/Multiple,GHC rejects valid program,,,,,