fix: subst notation (heq ▸ h) should fail if it does't subst #3994
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The subst notation substitues in the expected type, if present, or in the type of the argument, if no expected type is known. If there is an expected type it already fails if it cannot find the equations' left hand side or right hand side. But if the expected type is not known and the equation's lhs is not present in the second argument's type, it will happily do a no-op-substitution. This is inconsistent and unlikely what the user intended to do, so we now print an error message now. This still only looks for the lhs; search for the rhs as well seems prudent, but I’ll leave that for a separate PR, to better diagnose the impact on mathlib.
nomeata
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Apr 26, 2024
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| def ex4 (heq : foo = bar) (P : Nat → Prop) (h : P 3) := heq ▸ h -- error | ||
| def ex5 (heq : foo = bar) (P : Nat → Prop) (h : P 3) : P 4 := heq ▸ h -- error | ||
| def ex6 (heq : foo = bar) (P : Nat → Prop) (h : P 3) : P bar := heq ▸ h | ||
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| def ex7 (heq : bar = foo) (P : Nat → Prop) (h : P foo) := heq ▸ h -- error | ||
| def ex8 (heq : bar = foo) (P : Nat → Prop) (h : P foo) : P 4 := heq ▸ h -- error | ||
| def ex9 (heq : bar = foo) (P : Nat → Prop) (h : P foo) : P bar := heq ▸ h | ||
| def ex10 (heq : bar = foo) (P : Nat → Prop) (h : P 3) := heq ▸ h -- error | ||
| def ex11 (heq : bar = foo) (P : Nat → Prop) (h : P 3) : P 4 := heq ▸ h -- error | ||
| def ex12 (heq : bar = foo) (P : Nat → Prop) (h : P 3) : P bar := heq ▸ h |
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Previously, ex4, ex7 and ex10 would not err.
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refactor: avoid useless subst while working on leanprover/lean4#3994 I found a few uses of ▸ that do not actually do any rewriting. I plan to make this an error, but of course dropping them is already useful now.
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while working on leanprover/lean4#3994 I found a few uses of ▸ that do not actually do any rewriting. I plan to make this an error, but of course dropping them is already useful now.
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I've merged |
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The subst notation substitues in the expected type, if present, or in
the type of the argument, if no expected type is known.
If there is an expected type it already fails if it cannot find the
equations' left hand side or right hand side. But if the expected type
is not known and the equation's lhs is not present in the second
argument's type, it will happily do a no-op-substitution.
This is inconsistent and unlikely what the user intended to do, so we
now print an error message now.
This still only looks for the lhs; search for the rhs as well seems
prudent, but I’ll leave that for a separate PR, to better diagnose the
impact on mathlib.
This triggers a small number of pointless uses of subst in mathlib, see
leanprover-community/mathlib4#12451