Documentation

Lean.Elab.StructInst

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    Expand field abbreviations. Example: { x, y := 0 } expands to { x := x, y := 0 }

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        • Lean.Elab.Term.StructInst.instInhabitedField = { default := { ref := default, lhs := default, val := default, expr? := default } }
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            true iff all fields of the given structure are marked as default

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                                  • allStructNames : Array Lake.Name
                                  • maxDistance : Nat

                                    Consider the following example:

                                    structure A where
                                      x : Nat := 1
                                    
                                    structure B extends A where
                                      y : Nat := x + 1
                                      x := y + 1
                                    
                                    structure C extends B where
                                      z : Nat := 2*y
                                      x := z + 3
                                    

                                    And we are trying to elaborate a structure instance for C. There are default values for x at A, B, and C. We say the default value at C has distance 0, the one at B distance 1, and the one at A distance 2. The field maxDistance specifies the maximum distance considered in a round of Default field computation. Remark: since C does not set a default value of y, the default value at B is at distance 0.

                                    The fixpoint for setting default values works in the following way.

                                    • Keep computing default values using maxDistance == 0.
                                    • We increase maxDistance whenever we failed to compute a new default value in a round.
                                    • If maxDistance > 0, then we interrupt a round as soon as we compute some default value. We use depth-first search.
                                    • We sign an error if no progress is made when maxDistance == structure hierarchy depth (2 in the example above).
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                                                Reduce default value. It performs beta reduction and projections of the given structures.

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