4dd6cead83
It was not a good idea to use heterogeneous equality as the default equality in Lean.
It creates the following problems.
- Heterogeneous equality does not propagate constraints in the elaborator.
For example, suppose that l has type (List Int), then the expression
l = nil
will not propagate the type (List Int) to nil.
- It is easy to write false. For example, suppose x has type Real, and the user
writes x = 0. This is equivalent to false, since 0 has type Nat. The elaborator cannot introduce
the coercion since x = 0 is a type correct expression.
Homogeneous equality does not suffer from the problems above.
We keep heterogeneous equality because it is useful for generating proof terms.
Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
11 lines
206 B
Text
11 lines
206 B
Text
Set: pp::colors
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Set: pp::unicode
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Assumed: N
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Assumed: a
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Assumed: b
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a = b
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a = b : Bool
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Set: lean::pp::implicit
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eq::explicit N a b
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eq::explicit (Type 2) (Type 1) (Type 1)
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eq::explicit Bool ⊤ ⊥
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