RFC: NLP interface

doc/conf.py

@@ -33,7 +33,7 @@
 
 # Add any Sphinx extension module names here, as strings. They can be extensions
 # coming with Sphinx (named 'sphinx.ext.*') or your custom ones.
-extensions = ['sphinx.ext.mathjax', 'juliadoc.julia', 'juliadoc.jlhelp']
+extensions = ['sphinx.ext.mathjax','sphinx.ext.graphviz','juliadoc.julia', 'juliadoc.jlhelp']
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']

doc/index.rst

@@ -26,5 +26,6 @@ Contents
     solvers.rst
     mipcallbacks.rst
     sdp.rst
+    nlp.rst
 
 

doc/nlp.rst

@@ -0,0 +1,162 @@
+---------------------
+Nonlinear Programming
+---------------------
+
+MathProgBase provides an interface for nonlinear programming which is independent of both the solver and the user's representation of the problem, whether using an algebraic modeling language or customized low-level code.
+
+The diagram below illustrates MathProgBase as the connection between typical NLP solvers IPOPT, MOSEK, and KNITRO, and modeling languages such as `JuMP <https://github.com/JuliaOpt/JuMP.jl>`_ and `AMPL <http://ampl.com/>`_ (via `ampl.jl <https://github.com/dpo/ampl.jl>`_).
+
+.. graph:: foo
+
+   node [shape="box"];
+
+   subgraph clusterA {
+
+    "IPOPT" -- "MOSEK" -- "KNITRO" -- "..." [style="invis", constraint="false"];
+    label="Solvers";
+    penwidth=0;
+
+    }
+    "IPOPT" -- "JuMP" [style="invis"];
+   "IPOPT" -- "MathProgBase";
+   "MOSEK" -- "MathProgBase";
+   "KNITRO" -- "MathProgBase";
+   "..." -- "MathProgBase";
+   "MathProgBase" -- "JuMP";
+   "MathProgBase" -- "AMPL";
+   "MathProgBase" -- "User";
+   "MathProgBase" -- x;
+
+    subgraph clusterB {
+    x [label="..."];
+    "JuMP" -- "AMPL" -- "User" -- x [style="invis", constraint="false"];
+    label="Modeling";
+    penwidth=0;
+    }
+    rankdir=LR;
+
+This structure also makes it easy to connect solvers written in Julia itself with user-provided instances in a variety of formats.
+
+We take the prototypical format for a nonlinear problem as
+
+.. math::
+    \min_{x}\, &f(x)\\
+    s.t.     &lb \leq g(x) \leq ub\\
+             &l \leq x \leq u\\
+
+Where :math:`x \in \mathbb{R}^n, f: \mathbb{R}^n \to \mathbb{R}, g: \mathbb{R}^n \to \mathbb{R}^m`, and vectors :math:`lb \in \mathbb{R}^m \cup \{-\infty\}, ub \in \mathbb{R}^m \cup \{\infty\},l \in \mathbb{R}^n \cup \{-\infty\}, u \in \mathbb{R}^n \cup \{\infty\}`.
+
+The objective function :math:`f` and constraint function :math:`g` may be nonlinear and nonconvex, but are typically expected to be twice differentiable.
+
+Below we describe extensions to the ``MathProgSolverInterface`` for these nonlinear programming problems.
+
+.. function:: loadnonlinearproblem!(m::AbstractMathProgModel, numVar, numConstr, l, u, lb, ub, d::AbstractNLPEvaluator)
+
+    Loads the nonlinear programming problem into the model. The parameter `numVar` is the number of variables in the problem, ``numConstr`` is the number of constraints, ``l`` contains the variable lower bounds, ``u`` contains the variable upper bounds, ``lb`` contains the constraint lower bounds, and ``ub`` contains the constraint upper bounds. The final parameter ``d`` is an instance of an ``AbstractNLPEvaluator``, described below, which may be queried for evaluating :math:`f` and :math:`g` and their corresponding derivatives.
+
+The abstract type ``AbstractNLPEvaluator`` is used by solvers for accessing the objective function :math:`f` and constraints :math:`g`. Solvers may query the value, gradients, Hessian-vector products, and the Hessian of the Lagrangian.
+
+.. function:: initialize(d::AbstractNLPEvaluator, requested_features::Vector{Symbol})
+
+    Must be called before any other methods. The vector ``requested_features``
+    lists features requested by the solver. These may include ``:Grad`` for gradients
+    of :math:`f`, ``:Jac`` for explicit Jacobians of :math:`g`, ``:JacVec`` for
+    Jacobian-vector products, ``:HessVec`` for Hessian-vector
+    and Hessian-of-Lagrangian-vector products, ``:Hess`` for full Hessians and
+    Hessian-of-Lagrangians, and ``:ExprGraph`` for expression graphs.
+
+.. function:: features_available(d::AbstractNLPEvaluator)
+
+    Returns the subset of features available for this problem instance, as a
+    list of symbols in the same format as in ``initialize``.
+
+.. function:: eval_f(d::AbstractNLPEvaluator, x)
+
+    Evaluate :math:`f(x)`, returning a scalar value.
+
+.. function:: eval_g(d::AbstractNLPEvaluator, g, x)
+
+    Evaluate :math:`g(x)`, storing the result in the vector ``g`` which
+    must be of the appropriate size.
+
+.. function:: eval_grad_f(d::AbstractNLPEvaluator, g, x)
+
+    Evaluate :math:`\nabla f(x)` as a dense vector, storing 
+    the result in the vector ``g`` which must be of the appropriate size.
+
+.. function:: jac_structure(d::AbstractNLPEvaluator)
+
+    Returns the sparsity structure of the Jacobian matrix :math:`J_g(x) = \left[ \begin{array}{c} \nabla g_1(x) \\ \nabla g_2(x) \\ \vdots \\ \nabla g_m(x) \end{array}\right]` where :math:`g_i` is the :math:`i\text{th}` component of :math:`g`. The sparsity structure
+    is assumed to be independent of the point :math:`x`. Returns a tuple ``(I,J)``
+    where ``I`` contains the row indices and ``J`` contains the column indices of each
+    structurally nonzero element. These indices may not be sorted and can contain
+    duplicates, in which case the solver should combine the corresponding elements by
+    adding them together.
+
+.. function:: hesslag_structure(d::AbstractNLPEvaluator)
+
+    Returns the sparsity structure of the Hessian-of-the-Lagrangian matrix 
+    :math:`\nabla^2 f + \sum_{i=1}^m \nabla^2 g_i` as a tuple ``(I,J)``
+    where ``I`` contains the row indices and ``J`` contains the column indices of each
+    structurally nonzero element. These indices may not be sorted and can contain
+    duplicates, in which case the solver should combine the corresponding elements by
+    adding them together. Any mix of lower and upper-triangular indices is valid.
+    Elements ``(i,j)`` and ``(j,i)``, if both present, should be treated as duplicates.
+
+.. function:: eval_jac_g(d::AbstractNLPEvaluator, J, x)
+
+    Evaluates the sparse Jacobian matrix :math:`J_g(x) = \left[ \begin{array}{c} \nabla g_1(x) \\ \nabla g_2(x) \\ \vdots \\ \nabla g_m(x) \end{array}\right]`.
+    The result is stored in the vector ``J`` in the same order as the indices returned
+    by ``jac_structure``.
+
+.. function:: eval_jac_prod(d::AbstractNLPEvaluator, y, x, w)
+
+    Computes the Jacobian-vector product :math:`J_g(x)w`,
+    storing the result in the vector ``y``.
+
+.. function:: eval_jac_prod_t(d::AbstractNLPEvaluator, y, x, w)
+
+    Computes the Jacobian-transpose-vector product :math:`J_g(x)^Tw`,
+    storing the result in the vector ``y``.
+
+.. function:: eval_hesslag_prod(d::AbstractNLPEvaluator, h, x, v, σ, μ)
+
+    Given scalar weight ``σ`` and vector of constraint weights ``μ``, 
+    computes the Hessian-of-the-Lagrangian-vector product 
+    :math:`\left(\sigma\nabla^2 f(x) + \sum_{i=1}^m \mu_i \nabla^2 g_i(x)\right)v`, 
+    storing the result in the vector ``h``.
+
+.. function:: eval_hesslag(d::AbstractNLPEvaluator, H, x, σ, μ)
+
+    Given scalar weight ``σ`` and vector of constraint weights ``μ``, 
+    computes the sparse Hessian-of-the-Lagrangian matrix 
+    :math:`\sigma\nabla^2 f(x) + \sum_{i=1}^m \mu_i \nabla^2 g_i(x)`, 
+    storing the result in the vector ``H`` in the same order as the indices
+    returned by ``hesslag_structure``.
+
+.. function:: isobjlinear(d::AbstractNLPEvaluator)
+
+    ``true`` if the objective function is known to be linear,
+    ``false`` otherwise.
+
+.. function:: isobjquadratic(d::AbstractNLPEvaluator)
+
+    ``true`` if the objective function is known to be quadratic (convex or nonconvex),
+    ``false`` otherwise.
+
+.. function:: isconstrlinear(d::AbstractNLPEvaluator, i)
+
+    ``true`` if the :math:`i\text{th}` constraint is known to be linear,
+    ``false`` otherwise.
+
+.. function:: obj_expr(d::AbstractNLPEvaluator)
+
+    Returns an expression graph for the objective function. *FORMAT TO BE DETERMINED*
+
+.. function:: constr_expr(d::AbstractNLPEvaluator, i)
+
+    Returns an expression graph for the :math:`i\text{th}` constraint. *FORMAT TO BE DETERMINED*
+
+
+The solution vector, optimal objective value, termination status, etc. should be accessible from the standard methods, e.g., ``getsolution``, ``getobjval``, ``status``, respectively.
+