@keywords a, of, is, has, prefix .

@prefix : <http://yosi.us/lambda#> . 
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix implies: <http://www.w3.org/2000/10/swap/log#implies> .

#
# The evaluator.
#   The evaluator is a classic eval-apply loop
#
# we need a representation of functions
# a function is represented as
# (function <variable> <code> <environment>)
#
# an environment is represented as
# ((<variable> <value>) (<variable> <value>) ... )
#
#
#

#
# environment bindings
#
{(?X ?E) a NeedsFindBinding .
 ?E rdf:first ?L . # (?X ?B) 
 ?L rdf:first ?X; rdf:rest ?L2 .
 ?L2 rdf:first ?B; rdf:rest rdf:nil .
 ?E rdf:rest ?R .
}
=>
{(?X ?E) bindsThunked ?B .
 ?B a NeedsUnthunk } .

#case 5
{(thunk ?A ?E) a NeedsUnthunk}
=>
{(?A ?E) a NeedsEval} .

{(thunk ?A ?E) a NeedsUnthunk .
 (?A ?E) eval ?R .
}
=>
{(thunk ?A ?E) unthunk ?R} .

{(?X ?E) bindsThunked ?B .
 ?B unthunk ?R .
}
=>
{(?X ?E) binds ?R} .

{(?X ?E) a NeedsFindBinding .
 ?E rdf:rest ?R .
}
=>
{(?X ?R) a NeedsFindBinding} .

{(?X ?E) a NeedsFindBinding .
 ?E rdf:rest ?R .
 (?X ?R) binds ?B .
}
=>
{(?X ?E) binds ?B } .
#
# eval
# every eval is of the form
# (<expression> <env>) a NeedsEval
# <expression> is either a 
# 1.  lambda statement
# 2.  variable
# 3.  existing function
# 4.  application


#case 1.
{((lambda (?X) ?Y) ?E) a NeedsEval .
}
=>
{((lambda (?X) ?Y) ?E) eval (function ?X ?Y ?E) } .


#case 2
{(?X ?E) a NeedsEval}
=>
{(?X ?E) a NeedsFindBinding} .

{(?X ?E) a NeedsEval;
         binds ?R}
=>
{(?X ?E) eval ?R } .

#case 3
{((function ?X ?Y ?E) ?F) a NeedsEval}
=>
{((function ?X ?Y ?E) ?F) eval (function ?X ?Y ?E)} .

#case 4
{((?A ?B) ?E) a NeedsEval}
=>
{(?A ?E) a NeedsEval .
# (?B ?E) a NeedsEval 
} .

{((?A ?B) ?E) a NeedsEval .
 (?A ?E) eval ?F .
# (?B ?E) eval ?X 
}
=>
{(?F (thunk ?B ?E)) a NeedsApply} .

{((?A ?B) ?E) a NeedsEval .
 (?A ?E) eval ?F .
# (?B ?E) eval ?X .
 (?F (thunk ?B ?E)) apply ?R }
=>
{((?A ?B) ?E) eval ?R} .


#
# The apply part of the loop
#  
#
{((function ?X ?Y ?E) ?A) a NeedsApply}
=>
{(?Y [ rdf:first (?X ?A); rdf:rest ?E]) a NeedsEval } .

{((function ?X ?Y ?E) ?A) a NeedsApply .
 (?Y [ rdf:first (?X ?A); rdf:rest ?E]) eval ?R }
=>
{ ((function ?X ?Y ?E) ?A) apply ?R } .


#
# convienence
#
{?X a NeedsRun}
=>
{(?X ()) a NeedsEval} .

{?X a NeedsRun .
 (?X ()) eval ?R } 
=>
{?X results ?R } .