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pragmatic note on matching against decidable equality

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Peter Thiemann 2020-04-21 15:05:52 +02:00
parent 6ba26bbfc8
commit 8b9fc489c3
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src/plfa/part2/Properties.lagda.md
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@ -775,6 +775,14 @@ Where the construct introduces a bound variable we need to compare it
with the substituted variable, applying the drop lemma if they are with the substituted variable, applying the drop lemma if they are
equal and the swap lemma if they are distinct. equal and the swap lemma if they are distinct.
Testing variable names for equality using the `_≟_` operator has a
catch. For Agda it makes a difference whether we write `x ≟ y` or
`y ≟ x`. If the test matches the constraint that Agda expects, then the
proof goes through as demonstrated. Otherwise, Agda produces a message
that reveals the low-level implementation of string equality. To avoid
this problem, the guideline is to always match literally on the expression
on which Agda's evaluation is currently stuck.
#### Exercise `subst′` (stretch) #### Exercise `subst′` (stretch)
Rewrite `subst` to work with the modified definition `_[_:=_]′` Rewrite `subst` to work with the modified definition `_[_:=_]′`