mirror of https://github.com/golang/go.git
spec: update section on type inference for Go 1.21
The new section describes type inference as the problem of solving a set of type equations for bound type parameters. The next CL will update the section on unification to match the new inference approach. Change-Id: I2cb49bfb588ccc82d645343034096a82b7d602e2 Reviewed-on: https://go-review.googlesource.com/c/go/+/503920 TryBot-Bypass: Robert Griesemer <gri@google.com> Reviewed-by: Ian Lance Taylor <iant@google.com> Reviewed-by: Robert Griesemer <gri@google.com> Auto-Submit: Robert Griesemer <gri@google.com>
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Robert Griesemer
Gopher Robot
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doc/go_spec.html
484
doc/go_spec.html
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@ -1,6 +1,6 @@
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<!--{
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"Title": "The Go Programming Language Specification",
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"Subtitle": "Version of July 19, 2023",
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"Subtitle": "Version of July 20, 2023",
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"Path": "/ref/spec"
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}-->
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@ -2511,7 +2511,7 @@ type (
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<p>
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A type definition creates a new, distinct type with the same
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<a href="#Types">underlying type</a> and operations as the given type
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<a href="#Underlying_types">underlying type</a> and operations as the given type
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and binds an identifier, the <i>type name</i>, to it.
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</p>
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@ -4343,7 +4343,7 @@ type parameter list type arguments after substitution
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When using a generic function, type arguments may be provided explicitly,
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or they may be partially or completely <a href="#Type_inference">inferred</a>
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from the context in which the function is used.
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Provided that they can be inferred, type arguments may be omitted entirely if the function is:
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Provided that they can be inferred, type argument lists may be omitted entirely if the function is:
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</p>
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<ul>
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@ -4351,7 +4351,7 @@ Provided that they can be inferred, type arguments may be omitted entirely if th
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<a href="#Calls">called</a> with ordinary arguments,
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</li>
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<li>
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<a href="#Assignment_statements">assigned</a> to a variable with an explicitly declared type,
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<a href="#Assignment_statements">assigned</a> to a variable with a known type
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</li>
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<li>
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<a href="#Calls">passed as an argument</a> to another function, or
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@ -4371,7 +4371,7 @@ must be inferrable from the context in which the function is used.
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// sum returns the sum (concatenation, for strings) of its arguments.
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func sum[T ~int | ~float64 | ~string](x... T) T { … }
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x := sum // illegal: sum must have a type argument (x is a variable without a declared type)
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x := sum // illegal: the type of x is unknown
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intSum := sum[int] // intSum has type func(x... int) int
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a := intSum(2, 3) // a has value 5 of type int
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b := sum[float64](2.0, 3) // b has value 5.0 of type float64
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@ -4406,71 +4406,223 @@ For a generic type, all type arguments must always be provided explicitly.
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<h3 id="Type_inference">Type inference</h3>
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<p>
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<em>NOTE: This section is not yet up-to-date for Go 1.21.</em>
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A use of a generic function may omit some or all type arguments if they can be
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<i>inferred</i> from the context within which the function is used, including
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the constraints of the function's type parameters.
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Type inference succeeds if it can infer the missing type arguments
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and <a href="#Instantiations">instantiation</a> succeeds with the
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inferred type arguments.
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Otherwise, type inference fails and the program is invalid.
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</p>
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<p>
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Missing function type arguments may be <i>inferred</i> by a series of steps, described below.
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Each step attempts to use known information to infer additional type arguments.
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Type inference stops as soon as all type arguments are known.
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After type inference is complete, it is still necessary to substitute all type arguments
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for type parameters and verify that each type argument
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<a href="#Implementing_an_interface">implements</a> the relevant constraint;
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it is possible for an inferred type argument to fail to implement a constraint, in which
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case instantiation fails.
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Type inference uses the type relationships between pairs of types for inference:
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For instance, a function argument must be <a href="#Assignability">assignable</a>
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to its respective function parameter; this establishes a relationship between the
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type of the argument and the type of the parameter.
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If either of these two types contains type parameters, type inference looks for the
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type arguments to substitute the type parameters with such that the assignability
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relationship is satisfied.
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Similarly, type inference uses the fact that a type argument must
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<a href="#Satisfying_a_type_constraint">satisfy</a> the constraint of its respective
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type parameter.
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</p>
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<p>
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Type inference is based on
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Each such pair of matched types corresponds to a <i>type equation</i> containing
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one or multiple type parameters, from one or possibly multiple generic functions.
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Inferring the missing type arguments means solving the resulting set of type
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equations for the respective type parameters.
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</p>
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<p>
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For example, given
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</p>
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<pre>
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// dedup returns a copy of the argument slice with any duplicate entries removed.
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func dedup[S ~[]E, E comparable](S) S { … }
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type Slice []int
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var s Slice
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s = dedup(s) // same as s = dedup[Slice, int](s)
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</pre>
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<p>
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the variable <code>s</code> of type <code>Slice</code> must be assignable to
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the function parameter type <code>S</code> for the program to be valid.
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To reduce complexity, type inference ignores the directionality of assignments,
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so the type relationship between <code>Slice</code> and <code>S</code> can be
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expressed via the (symmetric) type equation <code>Slice ≡<sub>A</sub> S</code>
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(or <code>S ≡<sub>A</sub> Slice</code> for that matter),
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where the <code><sub>A</sub></code> in <code>≡<sub>A</sub></code>
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indicates that the LHS and RHS types must match per assignability rules
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(see the section on <a href="#Type_unification">type unifcation</a> for
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details).
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Similarly, the type parameter <code>S</code> must satisfy its constraint
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<code>~[]E</code>. This can be expressed as <code>S ≡<sub>C</sub> ~[]E</code>
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where <code>X ≡<sub>C</sub> Y</code> stands for
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"<code>X</code> satisfies constraint <code>Y</code>".
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These observations lead to a set of two equations
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</p>
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<pre>
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Slice ≡<sub>A</sub> S (1)
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S ≡<sub>C</sub> ~[]E (2)
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</pre>
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<p>
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which now can be solved for the type parameters <code>S</code> and <code>E</code>.
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From (1) a compiler can infer that the type argument for <code>S</code> is <code>Slice</code>.
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Similarly, because the underlying type of <code>Slice</code> is <code>[]int</code>
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and <code>[]int</code> must match <code>[]E</code> of the constraint,
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a compiler can infer that <code>E</code> must be <code>int</code>.
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Thus, for these two equations, type inference infers
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</p>
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<pre>
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S ➞ Slice
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E ➞ int
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</pre>
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<p>
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Given a set of type equations, the type parameters to solve for are
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the type parameters of the functions that need to be instantiated
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and for which no explicit type arguments is provided.
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These type parameters are called <i>bound</i> type parameters.
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For instance, in the <code>dedup</code> example above, the type parameters
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<code>P</code> and <code>E</code> are bound to <code>dedup</code>.
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An argument to a generic function call may be a generic function itself.
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The type parameters of that function are included in the set of bound
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type parameters.
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The types of function arguments may contain type parameters from other
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functions (such as a generic function enclosing a function call).
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Those type parameters may also appear in type equations but they are
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not bound in that context.
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Type equations are always solved for the bound type parameters only.
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</p>
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<p>
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Type inference supports calls of generic functions and assignments
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of generic functions to (explicitly function-typed) variables.
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This includes passing generic functions as arguments to other
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(possibly also generic) functions, and returning generic functions
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as results.
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Type inference operates on a set of equations specific to each of
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these cases.
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The equations are as follows (type argument lists are omitted for clarity):
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</p>
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<ul>
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<li>
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a <a href="#Type_parameter_declarations">type parameter list</a>
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<p>
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For a function call <code>f(a<sub>0</sub>, a<sub>1</sub>, …)</code> where
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<code>f</code> or a function argument <code>a<sub>i</sub></code> is
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a generic function:
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<br>
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Each pair <code>(a<sub>i</sub>, p<sub>i</sub>)</code> of corresponding
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function arguments and parameters where <code>a<sub>i</sub></code> is not an
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<a href="#Constants">untyped constant</a> yields an equation
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<code>typeof(p<sub>i</sub>) ≡<sub>A</sub> typeof(a<sub>i</sub>)</code>.
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<br>
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If <code>a<sub>i</sub></code> is an untyped constant <code>c<sub>j</sub></code>,
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and <code>p<sub>i</sub></code> is a bound type parameter <code>P<sub>k</sub></code>,
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the pair <code>(c<sub>j</sub>, P<sub>k</sub>)</code> is collected separately from
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the type equations.
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</p>
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</li>
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<li>
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a substitution map <i>M</i> initialized with the known type arguments, if any
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<p>
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For an assignment <code>v = f</code> of a generic function <code>f</code> to a
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(non-generic) variable <code>v</code> of function type:
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<br>
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<code>typeof(v) ≡<sub>A</sub> typeof(f)</code>.
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</p>
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</li>
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<li>
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a (possibly empty) list of ordinary function arguments (in case of a function call only)
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<p>
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For a return statement <code>return …, f, … </code> where <code>f</code> is a
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generic function returned as a result to a (non-generic) result variable
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of function type:
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<br>
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<code>typeof(r) ≡<sub>A</sub> typeof(f)</code>.
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</p>
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</li>
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</ul>
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<p>
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and then proceeds with the following steps:
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Additionally, each type parameter <code>P<sub>k</sub></code> and corresponding type constraint
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<code>C<sub>k</sub></code> yields the type equation
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<code>P<sub>k</sub> ≡<sub>C</sub> C<sub>k</sub></code>.
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</p>
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<p>
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Type inference gives precedence to type information obtained from typed operands
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before considering untyped constants.
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Therefore, inference proceeds in two phases:
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</p>
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<ol>
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<li>
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apply <a href="#Function_argument_type_inference"><i>function argument type inference</i></a>
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to all <i>typed</i> ordinary function arguments
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<p>
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The type equations are solved for the bound
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type parameters using <a href="#Type_unification">type unification</a>.
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If unification fails, type inference fails.
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</p>
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</li>
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<li>
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apply <a href="#Constraint_type_inference"><i>constraint type inference</i></a>
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</li>
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<li>
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apply function argument type inference to all <i>untyped</i> ordinary function arguments
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using the default type for each of the untyped function arguments
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</li>
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<li>
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apply constraint type inference
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<p>
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For each bound type parameter <code>P<sub>k</sub></code> for which no type argument
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has been inferred yet and for which one or more pairs
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<code>(c<sub>j</sub>, P<sub>k</sub>)</code> with that same type parameter
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were collected, determine the <a href="#Constant_expressions">constant kind</a>
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of the constants <code>c<sub>j</sub></code> in all those pairs the same way as for
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<a href="#Constant_expressions">constant expressions</a>.
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The type argument for <code>P<sub>k</sub></code> is the
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<a href="#Constants">default type</a> for the determined constant kind.
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If a constant kind cannot be determined due to conflicting constant kinds,
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type inference fails.
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</p>
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</li>
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</ol>
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<p>
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If there are no ordinary or untyped function arguments, the respective steps are skipped.
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Constraint type inference is skipped if the previous step didn't infer any new type arguments,
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but it is run at least once if there are missing type arguments.
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If not all type arguments have been found after these two phases, type inference fails.
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</p>
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<p>
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The substitution map <i>M</i> is carried through all steps, and each step may add entries to <i>M</i>.
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The process stops as soon as <i>M</i> has a type argument for each type parameter or if an inference step fails.
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If an inference step fails, or if <i>M</i> is still missing type arguments after the last step, type inference fails.
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If the two phases are successful, type inference determined a type argument for each
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bound type parameter:
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</p>
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<pre>
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P<sub>k</sub> ➞ A<sub>k</sub>
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</pre>
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<p>
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A type argument <code>A<sub>k</sub></code> may be a composite type,
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containing other bound type parameters <code>P<sub>k</sub></code> as element types
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(or even be just another bound type parameter).
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In a process of repeated simplification, the bound type parameters in each type
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argument are substituted with the respective type arguments for those type
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parameters until each type argument is free of bound type parameters.
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</p>
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<p>
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If type arguments contain cyclic references to themselves
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through bound type parameters, simplification and thus type
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inference fails.
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Otherwise, type inference succeeds.
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</p>
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<h4 id="Type_unification">Type unification</h4>
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<p>
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<em>
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Note: This section is not up-to-date for Go 1.21.
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</em>
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</p>
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<p>
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Type inference is based on <i>type unification</i>. A single unification step
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applies to a <a href="#Type_inference">substitution map</a> and two types, either
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@ -4546,264 +4698,6 @@ and the type literal <code>[]E</code>, unification compares <code>[]float64</cod
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the substitution map.
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</p>
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<h4 id="Function_argument_type_inference">Function argument type inference</h4>
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<!-- In this section and the section on constraint type inference we start with examples
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rather than have the examples follow the rules as is customary elsewhere in spec.
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Hopefully this helps building an intuition and makes the rules easier to follow. -->
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<p>
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Function argument type inference infers type arguments from function arguments:
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if a function parameter is declared with a type <code>T</code> that uses
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type parameters,
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<a href="#Type_unification">unifying</a> the type of the corresponding
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function argument with <code>T</code> may infer type arguments for the type
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parameters used by <code>T</code>.
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</p>
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<p>
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For instance, given the generic function
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</p>
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<pre>
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func scale[Number ~int64|~float64|~complex128](v []Number, s Number) []Number
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</pre>
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<p>
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and the call
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</p>
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<pre>
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var vector []float64
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scaledVector := scale(vector, 42)
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</pre>
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<p>
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the type argument for <code>Number</code> can be inferred from the function argument
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<code>vector</code> by unifying the type of <code>vector</code> with the corresponding
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parameter type: <code>[]float64</code> and <code>[]Number</code>
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match in structure and <code>float64</code> matches with <code>Number</code>.
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This adds the entry <code>Number</code> → <code>float64</code> to the
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<a href="#Type_unification">substitution map</a>.
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Untyped arguments, such as the second function argument <code>42</code> here, are ignored
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in the first round of function argument type inference and only considered if there are
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unresolved type parameters left.
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</p>
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<p>
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Inference happens in two separate phases; each phase operates on a specific list of
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(parameter, argument) pairs:
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</p>
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<ol>
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<li>
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The list <i>Lt</i> contains all (parameter, argument) pairs where the parameter
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type uses type parameters and where the function argument is <i>typed</i>.
|
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</li>
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<li>
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The list <i>Lu</i> contains all remaining pairs where the parameter type is a single
|
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type parameter. In this list, the respective function arguments are untyped.
|
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</li>
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</ol>
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<p>
|
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Any other (parameter, argument) pair is ignored.
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</p>
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|
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<p>
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By construction, the arguments of the pairs in <i>Lu</i> are <i>untyped</i> constants
|
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(or the untyped boolean result of a comparison). And because <a href="#Constants">default types</a>
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of untyped values are always predeclared non-composite types, they can never match against
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a composite type, so it is sufficient to only consider parameter types that are single type
|
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parameters.
|
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</p>
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|
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<p>
|
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Each list is processed in a separate phase:
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</p>
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||||
|
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<ol>
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<li>
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In the first phase, the parameter and argument types of each pair in <i>Lt</i>
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are unified. If unification succeeds for a pair, it may yield new entries that
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are added to the substitution map <i>M</i>. If unification fails, type inference
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fails.
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</li>
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<li>
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The second phase considers the entries of list <i>Lu</i>. Type parameters for
|
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which the type argument has already been determined are ignored in this phase.
|
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For each remaining pair, the parameter type (which is a single type parameter) and
|
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the <a href="#Constants">default type</a> of the corresponding untyped argument is
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unified. If unification fails, type inference fails.
|
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</li>
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</ol>
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|
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<p>
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While unification is successful, processing of each list continues until all list elements
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are considered, even if all type arguments are inferred before the last list element has
|
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been processed.
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</p>
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<p>
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Example:
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</p>
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|
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<pre>
|
||||
func min[T ~int|~float64](x, y T) T
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|
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var x int
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min(x, 2.0) // T is int, inferred from typed argument x; 2.0 is assignable to int
|
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min(1.0, 2.0) // T is float64, inferred from default type for 1.0 and matches default type for 2.0
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min(1.0, 2) // illegal: default type float64 (for 1.0) doesn't match default type int (for 2)
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</pre>
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|
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<p>
|
||||
In the example <code>min(1.0, 2)</code>, processing the function argument <code>1.0</code>
|
||||
yields the substitution map entry <code>T</code> → <code>float64</code>. Because
|
||||
processing continues until all untyped arguments are considered, an error is reported. This
|
||||
ensures that type inference does not depend on the order of the untyped arguments.
|
||||
</p>
|
||||
|
||||
<h4 id="Constraint_type_inference">Constraint type inference</h4>
|
||||
|
||||
<p>
|
||||
Constraint type inference infers type arguments by considering type constraints.
|
||||
If a type parameter <code>P</code> has a constraint with a
|
||||
<a href="#Core_types">core type</a> <code>C</code>,
|
||||
<a href="#Type_unification">unifying</a> <code>P</code> with <code>C</code>
|
||||
may infer additional type arguments, either the type argument for <code>P</code>,
|
||||
or if that is already known, possibly the type arguments for type parameters
|
||||
used in <code>C</code>.
|
||||
</p>
|
||||
|
||||
<p>
|
||||
For instance, consider the type parameter list with type parameters <code>List</code> and
|
||||
<code>Elem</code>:
|
||||
</p>
|
||||
|
||||
<pre>
|
||||
[List ~[]Elem, Elem any]
|
||||
</pre>
|
||||
|
||||
<p>
|
||||
Constraint type inference can deduce the type of <code>Elem</code> from the type argument
|
||||
for <code>List</code> because <code>Elem</code> is a type parameter in the core type
|
||||
<code>[]Elem</code> of <code>List</code>.
|
||||
If the type argument is <code>Bytes</code>:
|
||||
</p>
|
||||
|
||||
<pre>
|
||||
type Bytes []byte
|
||||
</pre>
|
||||
|
||||
<p>
|
||||
unifying the underlying type of <code>Bytes</code> with the core type means
|
||||
unifying <code>[]byte</code> with <code>[]Elem</code>. That unification succeeds and yields
|
||||
the <a href="#Type_unification">substitution map</a> entry
|
||||
<code>Elem</code> → <code>byte</code>.
|
||||
Thus, in this example, constraint type inference can infer the second type argument from the
|
||||
first one.
|
||||
</p>
|
||||
|
||||
<p>
|
||||
Using the core type of a constraint may lose some information: In the (unlikely) case that
|
||||
the constraint's type set contains a single <a href="#Type_definitions">defined type</a>
|
||||
<code>N</code>, the corresponding core type is <code>N</code>'s underlying type rather than
|
||||
<code>N</code> itself. In this case, constraint type inference may succeed but instantiation
|
||||
will fail because the inferred type is not in the type set of the constraint.
|
||||
Thus, constraint type inference uses the <i>adjusted core type</i> of
|
||||
a constraint: if the type set contains a single type, use that type; otherwise use the
|
||||
constraint's core type.
|
||||
</p>
|
||||
|
||||
<p>
|
||||
Generally, constraint type inference proceeds in two phases: Starting with a given
|
||||
substitution map <i>M</i>
|
||||
</p>
|
||||
|
||||
<ol>
|
||||
<li>
|
||||
For all type parameters with an adjusted core type, unify the type parameter with that
|
||||
type. If any unification fails, constraint type inference fails.
|
||||
</li>
|
||||
|
||||
<li>
|
||||
At this point, some entries in <i>M</i> may map type parameters to other
|
||||
type parameters or to types containing type parameters. For each entry
|
||||
<code>P</code> → <code>A</code> in <i>M</i> where <code>A</code> is or
|
||||
contains type parameters <code>Q</code> for which there exist entries
|
||||
<code>Q</code> → <code>B</code> in <i>M</i>, substitute those
|
||||
<code>Q</code> with the respective <code>B</code> in <code>A</code>.
|
||||
Stop when no further substitution is possible.
|
||||
</li>
|
||||
</ol>
|
||||
|
||||
<p>
|
||||
The result of constraint type inference is the final substitution map <i>M</i> from type
|
||||
parameters <code>P</code> to type arguments <code>A</code> where no type parameter <code>P</code>
|
||||
appears in any of the <code>A</code>.
|
||||
</p>
|
||||
|
||||
<p>
|
||||
For instance, given the type parameter list
|
||||
</p>
|
||||
|
||||
<pre>
|
||||
[A any, B []C, C *A]
|
||||
</pre>
|
||||
|
||||
<p>
|
||||
and the single provided type argument <code>int</code> for type parameter <code>A</code>,
|
||||
the initial substitution map <i>M</i> contains the entry <code>A</code> → <code>int</code>.
|
||||
</p>
|
||||
|
||||
<p>
|
||||
In the first phase, the type parameters <code>B</code> and <code>C</code> are unified
|
||||
with the core type of their respective constraints. This adds the entries
|
||||
<code>B</code> → <code>[]C</code> and <code>C</code> → <code>*A</code>
|
||||
to <i>M</i>.
|
||||
|
||||
<p>
|
||||
At this point there are two entries in <i>M</i> where the right-hand side
|
||||
is or contains type parameters for which there exists other entries in <i>M</i>:
|
||||
<code>[]C</code> and <code>*A</code>.
|
||||
In the second phase, these type parameters are replaced with their respective
|
||||
types. It doesn't matter in which order this happens. Starting with the state
|
||||
of <i>M</i> after the first phase:
|
||||
</p>
|
||||
|
||||
<p>
|
||||
<code>A</code> → <code>int</code>,
|
||||
<code>B</code> → <code>[]C</code>,
|
||||
<code>C</code> → <code>*A</code>
|
||||
</p>
|
||||
|
||||
<p>
|
||||
Replace <code>A</code> on the right-hand side of → with <code>int</code>:
|
||||
</p>
|
||||
|
||||
<p>
|
||||
<code>A</code> → <code>int</code>,
|
||||
<code>B</code> → <code>[]C</code>,
|
||||
<code>C</code> → <code>*int</code>
|
||||
</p>
|
||||
|
||||
<p>
|
||||
Replace <code>C</code> on the right-hand side of → with <code>*int</code>:
|
||||
</p>
|
||||
|
||||
<p>
|
||||
<code>A</code> → <code>int</code>,
|
||||
<code>B</code> → <code>[]*int</code>,
|
||||
<code>C</code> → <code>*int</code>
|
||||
</p>
|
||||
|
||||
<p>
|
||||
At this point no further substitution is possible and the map is full.
|
||||
Therefore, <code>M</code> represents the final map of type parameters
|
||||
to type arguments for the given type parameter list.
|
||||
</p>
|
||||
|
||||
<h3 id="Operators">Operators</h3>
|
||||
|
||||
<p>
|
||||
|
|
@ -5479,7 +5373,7 @@ in any of these cases:
|
|||
ignoring struct tags (see below),
|
||||
<code>x</code>'s type and <code>T</code> are not
|
||||
<a href="#Type_parameter_declarations">type parameters</a> but have
|
||||
<a href="#Type_identity">identical</a> <a href="#Types">underlying types</a>.
|
||||
<a href="#Type_identity">identical</a> <a href="#Underlying_types">underlying types</a>.
|
||||
</li>
|
||||
<li>
|
||||
ignoring struct tags (see below),
|
||||
|
|
@ -8291,7 +8185,7 @@ of if the general conversion rules take care of this.
|
|||
<p>
|
||||
A <code>Pointer</code> is a <a href="#Pointer_types">pointer type</a> but a <code>Pointer</code>
|
||||
value may not be <a href="#Address_operators">dereferenced</a>.
|
||||
Any pointer or value of <a href="#Types">underlying type</a> <code>uintptr</code> can be
|
||||
Any pointer or value of <a href="#Underlying_types">underlying type</a> <code>uintptr</code> can be
|
||||
<a href="#Conversions">converted</a> to a type of underlying type <code>Pointer</code> and vice versa.
|
||||
The effect of converting between <code>Pointer</code> and <code>uintptr</code> is implementation-defined.
|
||||
</p>
|
||||
|
|
|
|||
Loading…
Reference in New Issue