Cannot assign Maple function by directly using expression, only by referencing output

Question

Why does this does not work for assigning a function:

F(t) := Matrix(matrixDE(A, t)[1])

While using a label to reference the output works:

Matrix(matrixDE(A, t)[1])      (1)
F1(t) := (1)

matrixDE(A, t)[1]              (2)
F2(t) := Matrix((2))

It seems the indexing operation [1] is what poses the problem, but I don't understand the mechanic behind this.

I found this question but it did not make me any wiser...

---

Edit:

I ended up getting my desired effect using eval and :

F(x) := eval(Matrix(matrixDE(A, t)[1]), t = x)

The price to pay being that the argument of Fis named x instead of t.

Answer 1

The problem has nothing to with the indexing. The problem is exactly what you said in your title: You cannot (properly) assign a Maple function by directly using an (unevalauted) expression; rather, you must reference the output (or evaluation).

More specifically, it depends on the desired order of these two operations: evaluation of a parameterized expression and substitution of values for the parameter. If you want to first evaluate the expression with a symbolic parameter (your t in this case), then the command to use is unapply:

F:= unapply(Matrix(matrixDE(A, t)[1]), t);