A proposed compromise for the renaming PR

src/TermInterface.jl

@@ -3,7 +3,7 @@ module TermInterface
 """
   istree(x)
 
-Returns `true` if `x` is a term. If true, `operation`, `arguments`
+Returns `true` if `x` is a term. If true, `head` and `children`
 must also be defined for `x` appropriately.
 """
 istree(x) = false
@@ -12,14 +12,12 @@ export istree
 """
   symtype(x)
 
-Returns the symbolic type of `x`. By default this is just `typeof(x)`.
+Returns the symbolic type of `x`. By default this is just `Any`.
 Define this for your symbolic types if you want `SymbolicUtils.simplify` to apply rules
 specific to numbers (such as commutativity of multiplication). Or such
 rules that may be implemented in the future.
 """
-function symtype(x)
-  typeof(x)
-end
+symtype(x) = Any
 export symtype
 
 """
@@ -31,20 +29,6 @@ on `x` and must return a Symbol.
 issym(x) = false
 export issym
 
-"""
-  exprhead(x)
-
-If `x` is a term as defined by `istree(x)`, `exprhead(x)` must return a symbol,
-corresponding to the head of the `Expr` most similar to the term `x`.
-If `x` represents a function call, for example, the `exprhead` is `:call`.
-If `x` represents an indexing operation, such as `arr[i]`, then `exprhead` is `:ref`.
-Note that `exprhead` is different from `operation` and both functions should 
-be defined correctly in order to let other packages provide code generation 
-and pattern matching features. 
-"""
-function exprhead end
-export exprhead
-
 """
   head(x)
 
@@ -54,41 +38,37 @@ term if `x`. The `head` type has to be provided by the package.
 function head end
 export head
 
-"""
-  head_symbol(x::HeadType)
-
-If `x` is a head object, `head_symbol(T, x)` returns a `Symbol` object that
-corresponds to `y.head` if `y` was the representation of the corresponding term
-as a Julia Expression. This is useful to define interoperability between
-symbolic term types defined in different packages and should be used when
-calling `maketerm`.
-"""
-function head_symbol end
-export head_symbol
-
 """
   children(x)
 
-Get the arguments of `x`, must be defined if `istree(x)` is `true`.
+Get the children of `x`, must be defined if `istree(x)` is `true`.
 """
 function children end
 export children
 
+"""
+  is_function_call(x)
+
+Return true if `x` is a term as defined by `istree(x)` and corresponds to a
+function call. If true, `operation` and `arguments` must be defined
+appropriately.
+"""
+is_function_call(x) = false
 
 """
   operation(x)
 
-If `x` is a term as defined by `istree(x)`, `operation(x)` returns the
-operation of the term if `x` represents a function call, for example, the head
-is the function being called.
+If `x` is a function call as defined by `is_function_call(x)`, `operation(x)`
+returns the function being called.
 """
 function operation end
 export operation
 
 """
   arguments(x)
 
-Get the arguments of `x`, must be defined if `istree(x)` is `true`.
+If `x` is a function call as defined by `is_function_call(x)`, `arguments(x)`
+returns the arguments on which the function is called.
 """
 function arguments end
 export arguments
@@ -137,74 +117,17 @@ end
 
 
 """
-  maketerm(head::H, children; type=Any, metadata=nothing)
+  maketerm(head::H, children; symtype=Any, metadata=nothing)
 
-Has to be implemented by the provider of H.
-Returns a term that is in the same closure of types as `typeof(x)`,
-with `head` as the head and `children` as the arguments, `type` as the symtype
-and `metadata` as the metadata. 
+Has to be implemented by the provider of H.  Returns a term that is in the same
+closure of types as `H`, with `head` as the head and `children` as the children,
+`symtype` as the symtype and `metadata` as the metadata.
 """
 function maketerm end
 export maketerm
 
-"""
-  is_operation(f)
-
-Returns a single argument anonymous function predicate, that returns `true` if and only if
-the argument to the predicate satisfies `istree` and `operation(x) == f` 
-"""
-is_operation(f) = @nospecialize(x) -> istree(x) && (operation(x) == f)
-export is_operation
-
-
-"""
-  node_count(t)
-Count the nodes in a symbolic expression tree satisfying `istree` and `arguments`.
-"""
-node_count(t) = istree(t) ? reduce(+, node_count(x) for x in arguments(t), init = 0) + 1 : 1
-export node_count
-
+include("utils.jl")
 include("expr.jl")
 
-""" 
-  @matchable struct Foo fields... end [HeadType]
-
-Take a struct definition and automatically define `TermInterface` methods. This
-will automatically define a head type. If `HeadType` is given then it will be
-used as `head(::Foo)`. If it is omitted, and the struct is called `Foo`, then
-the head type will be called `FooHead`. The `head_symbol` of such head types
-will default to `:call`.
-"""
-macro matchable(expr, head_name=nothing)
-  @assert expr.head == :struct
-  name = expr.args[2]
-  if name isa Expr
-    name.head === :(<:) && (name = name.args[1])
-    name isa Expr && name.head === :curly && (name = name.args[1])
-  end
-  fields = filter(x -> x isa Symbol || (x isa Expr && x.head == :(::)), expr.args[3].args)
-  get_name(s::Symbol) = s
-  get_name(e::Expr) = (@assert(e.head == :(::)); e.args[1])
-  fields = map(get_name, fields)
-  head_name = isnothing(head_name) ? Symbol(name, :Head) : head_name
-
-  quote
-    $expr
-    struct $head_name
-      head
-    end
-    TermInterface.head_symbol(x::$head_name) = x.head
-    # TODO default to call?
-    TermInterface.head(::$name) = $head_name(:call)
-    TermInterface.istree(::$name) = true
-    TermInterface.operation(::$name) = $name
-    TermInterface.arguments(x::$name) = getfield.((x,), ($(QuoteNode.(fields)...),))
-    TermInterface.children(x::$name) = [operation(x); arguments(x)...]
-    TermInterface.arity(x::$name) = $(length(fields))
-    Base.length(x::$name) = $(length(fields) + 1)
-  end |> esc
-end
-export @matchable
-
 end # module
 

src/expr.jl

@@ -6,20 +6,20 @@ struct ExprHead
 end
 export ExprHead
 
-head_symbol(eh::ExprHead) = eh.head
-
 istree(x::Expr) = true
 head(e::Expr) = ExprHead(e.head)
 children(e::Expr) = e.args
 
+is_function_call(e::Expr) = head(e).head in (:call, :macrocall)
+
 # See https://docs.julialang.org/en/v1/devdocs/ast/
 function operation(e::Expr)
   h = head(e)
   hh = h.head
   if hh in (:call, :macrocall)
     e.args[1]
   else
-    hh
+    throw(ArgumentError("Not a function call"))
   end
 end
 
@@ -29,7 +29,7 @@ function arguments(e::Expr)
   if hh in (:call, :macrocall)
     e.args[2:end]
   else
-    e.args
+    throw(ArgumentError("Not a function call"))
   end
 end
 

src/utils.jl

@@ -0,0 +1,16 @@
+"""
+  is_head(f)
+
+Returns a single argument anonymous function predicate, that returns `true` if and only if
+the argument to the predicate satisfies `istree` and `head(x) == f` 
+"""
+is_head(f) = @nospecialize(x) -> istree(x) && (head(x) == f)
+export is_head
+
+
+"""
+  node_count(t)
+Count the nodes in a symbolic expression tree satisfying `istree` and `arguments`.
+"""
+node_count(t) = istree(t) ? reduce(+, node_count(x) for x in children(t), init = 0) + 1 : 1
+export node_count

test/runtests.jl

@@ -3,30 +3,29 @@ using TermInterface, Test
 @testset "Expr" begin
     ex = :(f(a, b))
     @test head(ex) == ExprHead(:call)
+    @test is_function_call(ex) == true
     @test children(ex) == [:f, :a, :b]
     @test operation(ex) == :f
     @test arguments(ex) == [:a, :b]
     @test ex == maketerm(ExprHead(:call), [:f, :a, :b])
 
     ex = :(arr[i, j])
     @test head(ex) == ExprHead(:ref)
-    @test operation(ex) == :ref
-    @test arguments(ex) == [:arr, :i, :j]
+    @test children(ex) == [:arr, :i, :j]
+    @test is_function_call(ex) == false
     @test ex == maketerm(ExprHead(:ref), [:arr, :i, :j])
 
 
     ex = :(i, j)
     @test head(ex) == ExprHead(:tuple)
-    @test operation(ex) == :tuple
-    @test arguments(ex) == [:i, :j]
     @test children(ex) == [:i, :j]
+    @test is_function_call(ex) == false
     @test ex == maketerm(ExprHead(:tuple), [:i, :j])
 
-
     ex = Expr(:block, :a, :b, :c)
     @test head(ex) == ExprHead(:block)
-    @test operation(ex) == :block
-    @test children(ex) == arguments(ex) == [:a, :b, :c]
+    @test children(ex) == [:a, :b, :c]
+    @test is_function_call(ex) == false
     @test ex == maketerm(ExprHead(:block), [:a, :b, :c])
 end
 
@@ -40,31 +39,12 @@ end
     end
     TermInterface.head(::Foo) = FooHead(:call)
     TermInterface.head_symbol(q::FooHead) = q.head
-    TermInterface.operation(::Foo) = Foo
     TermInterface.istree(::Foo) = true
-    TermInterface.arguments(x::Foo) = [x.args...]
-    TermInterface.children(x::Foo) = [operation(x); x.args...]
+    TermInterface.children(x::Foo) = x.args
 
     t = Foo(1, 2)
     @test head(t) == FooHead(:call)
     @test head_symbol(head(t)) == :call
-    @test operation(t) == Foo
-    @test istree(t) == true
-    @test arguments(t) == [1, 2]
-    @test children(t) == [Foo, 1, 2]
-end
-
-@testset "Automatically Generated Methods" begin
-    @matchable struct Bar
-        a
-        b::Int
-    end
-
-    t = Bar(1, 2)
-    @test head(t) == BarHead(:call)
-    @test head_symbol(head(t)) == :call
-    @test operation(t) == Bar
     @test istree(t) == true
-    @test arguments(t) == (1, 2)
-    @test children(t) == [Bar, 1, 2]
+    @test children(t) == [1, 2]
 end