make all maketerm arguments mandatory

Fixes #33

Author: nsajko

src/TermInterface.jl

@@ -111,7 +111,7 @@ end
 
 
 """
-    maketerm(T, head, children, type=nothing, metadata=nothing)
+    maketerm(T, head, children, type, metadata)
 
 Constructs an expression. `T` is a constructor type, `head` and `children` are
 the head and tail of the S-expression, `type` is the `type` of the S-expression.
@@ -127,11 +127,10 @@ the sub-expression. `T` will be the type of the outer expression.
 
 Packages providing expression types _must_ implement this method for each expression type.
 
-If your types do not support type information or metadata, you still need to accept
-these arguments and may choose to not use them.
+Giving `nothing` for `type` or `metadata` results in a default being selected.
 """
 
-function maketerm(T::Type, head, children, type=nothing, metadata=nothing)
+function maketerm(T::Type, head, children, type, metadata)
   error("maketerm for $T is not implemented")
 end
 export maketerm

src/expr.jl

@@ -10,6 +10,6 @@ children(e::Expr) = e.args
 operation(e::Expr) = iscall(e) ? first(children(e)) : error("operation called on a non-function call expression")
 arguments(e::Expr) = iscall(e) ? @view(e.args[2:end]) : error("arguments called on a non-function call expression")
 
-function maketerm(::Type{Expr}, head, args, type=nothing, metadata=nothing)
+function maketerm(::Type{Expr}, head, args, type, metadata)
     Expr(head, args...)
 end

test/runtests.jl

@@ -9,7 +9,7 @@ using Test
     @test arguments(ex) == [:a, :b]
     @test isexpr(ex)
     @test iscall(ex)
-    @test ex == maketerm(Expr, :call, [:f, :a, :b])
+    @test ex == maketerm(Expr, :call, [:f, :a, :b], nothing, nothing)
 
 
     ex = :(arr[i, j])
@@ -18,5 +18,5 @@ using Test
     @test_throws ErrorException arguments(ex)
     @test isexpr(ex)
     @test !iscall(ex)
-    @test ex == maketerm(Expr, :ref, [:arr, :i, :j])
+    @test ex == maketerm(Expr, :ref, [:arr, :i, :j], nothing, nothing)
 end