lang: unification: Add type inference state debugging

This presents things in a more formal way for those who are more
familiar with standard type unification syntax.

Patch created by Sam, and cleaned up by James.

lang/unification/simplesolver.go

@@ -19,6 +19,7 @@ package unification // TODO: can we put this solver in a sub-package?
 
 import (
 	"fmt"
+	"sort"
 
 	"github.com/purpleidea/mgmt/lang/interfaces"
 	"github.com/purpleidea/mgmt/lang/types"
@@ -57,6 +58,126 @@ func SimpleInvariantSolverLogger(logf func(format string, v ...interface{})) fun
 	}
 }
 
+// DebugSolverState helps us in understanding the state of the type unification
+// solver in a more mainstream format.
+// Example:
+//
+// solver state:
+//
+// *	str("foo") :: str
+// *	call:f(str("foo")) [0xc000ac9f10] :: ?1
+// *	var(x) [0xc00088d840] :: ?2
+// *	param(x) [0xc00000f950] :: ?3
+// *	func(x) { var(x) } [0xc0000e9680] :: ?4
+// *	?2 = ?3
+// *	?4 = func(arg0 str) ?1
+// *	?4 = func(x str) ?2
+// *	?1 = ?2
+func DebugSolverState(solved map[interfaces.Expr]*types.Type, equalities []interfaces.Invariant) string {
+	s := ""
+
+	// all the relevant Exprs
+	count := 0
+	exprs := make(map[interfaces.Expr]int)
+	for _, equality := range equalities {
+		for _, expr := range equality.ExprList() {
+			count++
+			exprs[expr] = count // for sorting
+		}
+	}
+
+	// print the solved Exprs first
+	for expr, typ := range solved {
+		s += fmt.Sprintf("%v :: %v\n", expr, typ)
+		delete(exprs, expr)
+	}
+
+	sortedExprs := []interfaces.Expr{}
+	for k := range exprs {
+		sortedExprs = append(sortedExprs, k)
+	}
+	sort.Slice(sortedExprs, func(i, j int) bool { return exprs[sortedExprs[i]] < exprs[sortedExprs[j]] })
+
+	// for each remaining expr, generate a shorter name than the full pointer
+	nextVar := 1
+	shortNames := map[interfaces.Expr]string{}
+	for _, expr := range sortedExprs {
+		shortNames[expr] = fmt.Sprintf("?%d", nextVar)
+		nextVar++
+		s += fmt.Sprintf("%p %v :: %s\n", expr, expr, shortNames[expr])
+	}
+
+	// print all the equalities using the short names
+	for _, equality := range equalities {
+		switch e := equality.(type) {
+		case *interfaces.EqualsInvariant:
+			_, ok := solved[e.Expr]
+			if !ok {
+				s += fmt.Sprintf("%s = %v\n", shortNames[e.Expr], e.Type)
+			} else {
+				// if solved, then this is redundant, don't print anything
+			}
+
+		case *interfaces.EqualityInvariant:
+			type1, ok1 := solved[e.Expr1]
+			type2, ok2 := solved[e.Expr2]
+			if !ok1 && !ok2 {
+				s += fmt.Sprintf("%s = %s\n", shortNames[e.Expr1], shortNames[e.Expr2])
+			} else if ok1 && !ok2 {
+				s += fmt.Sprintf("%s = %s\n", type1, shortNames[e.Expr2])
+			} else if !ok1 && ok2 {
+				s += fmt.Sprintf("%s = %s\n", shortNames[e.Expr1], type2)
+			} else {
+				// if completely solved, then this is redundant, don't print anything
+			}
+
+		case *interfaces.EqualityWrapFuncInvariant:
+			funcType, funcOk := solved[e.Expr1]
+
+			args := ""
+			argsOk := true
+			for i, argName := range e.Expr2Ord {
+				if i > 0 {
+					args += ", "
+				}
+				argExpr := e.Expr2Map[argName]
+				argType, ok := solved[argExpr]
+				if !ok {
+					args += fmt.Sprintf("%s %s", argName, shortNames[argExpr])
+					argsOk = false
+				} else {
+					args += fmt.Sprintf("%s %s", argName, argType)
+				}
+			}
+
+			outType, outOk := solved[e.Expr2Out]
+
+			if !funcOk || !argsOk || !outOk {
+				if !funcOk && !outOk {
+					s += fmt.Sprintf("%s = func(%s) %s\n", shortNames[e.Expr1], args, shortNames[e.Expr2Out])
+				} else if !funcOk && outOk {
+					s += fmt.Sprintf("%s = func(%s) %s\n", shortNames[e.Expr1], args, outType)
+				} else if funcOk && !outOk {
+					s += fmt.Sprintf("%s = func(%s) %s\n", funcType, args, shortNames[e.Expr2Out])
+				} else {
+					s += fmt.Sprintf("%s = func(%s) %s\n", funcType, args, outType)
+				}
+			}
+
+		case *interfaces.CallFuncArgsValueInvariant:
+			// skip, not used in the examples I care about
+
+		case *interfaces.AnyInvariant:
+			// skip, not used in the examples I care about
+
+		default:
+			s += fmt.Sprintf("%v\n", equality)
+		}
+	}
+
+	return s
+}
+
 // SimpleInvariantSolver is an iterative invariant solver for AST expressions.
 // It is intended to be very simple, even if it's computationally inefficient.
 func SimpleInvariantSolver(invariants []interfaces.Invariant, expected []interfaces.Expr, logf func(format string, v ...interface{})) (*InvariantSolution, error) {
@@ -876,6 +997,7 @@ Loop:
 					logf("%s: unsolved: %+v", Name, x)
 				}
 			}
+			logf("%s: solver state:\n%s", Name, DebugSolverState(solved, equalities))
 
 			// Lastly, we could loop through each exclusive and see
 			// if it only has a single, easy solution. For example,