Reduce allocations involved in temporary types?

[Isaac Elliott]

The instruction count / heap usage of my usual benchmark (./scripts/{callgrind,dhat} examples/todolist.ipso < ipso-cli/benches/todolist_1_input.txt) crept up a little bit in #263.

I suspect it's from the extra types I create during checking. Examples:

• ipso/ipso-typecheck/src/type_inference/mod.rs

  Lines 454 to 462 in c0fa357

   unification::unify( 
       env.as_unification_env(), 
       &mut state.kind_inference_state, 
       &mut state.type_solutions, 
       pos, 
       expected, 
       &Type::mk_app(&Type::mk_record_ctor(env.common_kinds), &entire_row), 
   ) 
   .map_err(|error| Error::unification_error(env.source, pos, error))?; 

• ipso/ipso-typecheck/src/type_inference/mod.rs

  Lines 512 to 524 in c0fa357

   unification::unify( 
       env.as_unification_env(), 
       &mut state.kind_inference_state, 
       &mut state.type_solutions, 
       pos, 
       expected, 
       &Type::mk_variant( 
           env.common_kinds, 
           vec![(ctor.clone(), arg_ty.clone())], 
           Some(rest_row.clone()), 
       ), 
   ) 
   .map_err(|error| Error::unification_error(env.source, pos, error))?; 

• ipso/ipso-typecheck/src/type_inference/mod.rs

  Lines 740 to 745 in c0fa357

   let fun = check( 
       env, 
       state, 
       fun, 
       &Type::mk_arrow(env.common_kinds, &in_ty, &out_ty), 
   )?; 

• ipso/ipso-typecheck/src/type_inference/mod.rs

  Lines 770 to 780 in c0fa357

   unification::unify( 
       env.as_unification_env(), 
       &mut state.kind_inference_state, 
       &mut state.type_solutions, 
       position, 
       expected, 
       &in_tys.iter().rev().fold(out_ty.clone(), |acc, el| { 
           Type::arrow(env.common_kinds, el.clone(), acc) 
       }), 
   ) 
   .map_err(|error| Error::unification_error(env.source, position, error))?; 

• ipso/ipso-typecheck/src/type_inference/mod.rs

  Lines 959 to 967 in c0fa357

   unification::unify( 
       env.as_unification_env(), 
       &mut state.kind_inference_state, 
       &mut state.type_solutions, 
       expr.pos, 
       expected, 
       &Type::app(Type::mk_array(env.common_kinds), item_type.clone()), 
   ) 
   .map_err(|error| Error::unification_error(env.source, expr.pos, error))?; 

and more.

These temporary types involve heap allocations, because they're recursive.

Is this a big deal and is there a cheap way to do it?