Codom error when using rule with attribute variables

[slwu89]

If I have an acset and one of the attribute types is being modeled as Vector{String} and I make a rewrite rule which is mapped to another Vector{String} as follows, I get the following error ERROR: Codom error: Any[Any[["start"], "rule"]] Vector{String} FinSet(0)

example:

using Catlab, AlgebraicRewriting

@present TestSch(FreeSchema) begin
    X::Ob
    MyAttr::AttrType
    myattr::Attr(X,MyAttr)
end

@acset_type TestData(TestSch)

data = @acset TestData{Vector{String}} begin
    X=1
    myattr=[["start"]]
end

L = TestData{Vector{String}}()
add_part!(L, :MyAttr)
add_part!(L, :X, myattr=AttrVar(1))

I = TestData{Vector{String}}()

R = deepcopy(L)

l = only(homomorphisms(I, L, monic=true))
r = only(homomorphisms(I, R, monic=true))

rule = Rule(l, r, expr=(MyAttr=[t -> vcat(t, "rule")], ))

m = get_matches(rule, data)
rewrite_match(rule, only(m))

[kris-brown]

so the anonymous function that is used to assign values in $R$ is a function of the bound variables in $L$. So t is actually a list! A singleton list, since there is only one MyAttr value in $L$, but it's still a list, so your code would work if you did vcat(only(t), "rule")

[slwu89]

Ah ok, thanks! So the length of the list for each AttrType will be the number of bound variables in $L$ for that attribute?

[slwu89]

@kris-brown posting another question here which is related. When trying to do the following rewrite I get an error related to computing the pushout complement. The specific example here is contrived, but it comes up when wanting to do a rewrite that updates a hom, but leaves all the other attributes/homs of something unchanged.

Also, happy to contribute to the docs related to rewriting with attribute variables after all this!

@present TestSch(FreeSchema) begin
    X::Ob
    Label::AttrType
    x1::Attr(X,Label)
    x2::Attr(X,Label)
end

@acset_type TestData(TestSch)

L = @acset TestData{Symbol} begin
    X=1
    Label=2
    x1=[AttrVar(1)]
    x2=[AttrVar(2)]
end

I = TestData{Symbol}()

R = @acset TestData{Symbol} begin
    X=1
    Label=2
    x1=[AttrVar(1)]
    x2=[AttrVar(2)]
end

l = only(homomorphisms(I, L, monic=true))
r = only(homomorphisms(I, R, monic=true))

rule = Rule(l, r, expr=(Label=[x1 -> first(x1), x2 -> last(x2)], ))

test = @acset TestData{Symbol} begin
    X=1
    Label=2
    x1=[Symbol("a")]
    x2=[Symbol("b")]
end

m = get_matches(rule, test)
test1 = rewrite_match(rule, m[1])

[kris-brown]

Here the error is Cannot rewrite with AttrVars in G which means that, in the graph you were trying to rewrite, there were variables. I think by accident you forgot to delete the Label=2 in test?

[slwu89]

Ah, user error!