Type inference lets the F# compiler determine symbol types from how values are defined and used, rather than requiring an annotation on every binding. F# remains statically typed: inferred code receives the same compiler checking while removing routine type declarations. section 3.2 section 3.2.1
Use it by default for values, functions, records, and composed operations. Less annotation makes code quicker to write and easier to read; when a refactor changes real type relationships, compiler errors identify the clashes instead of requiring boilerplate edits throughout the codebase. section 3.2.1 section 3.4
Inferred and explicit bindings
Both bindings represent an integer value; the annotation is optional when the right-hand side already establishes the type. An incompatible annotation produces a compiler error. let i = 10 let i : int = 10 section 3.2.2
Inference follows program constraints rather than guessing. If an unexpected early use makes a parameter appear to be a string, later numeric call sites can fail. Inspect the inferred signature, then add a temporary annotation at the intended boundary to move the error to the underlying mismatch. section 3.2.4 section 3.2.6
let calculateGroup age = #1
if age < 18 then "Child"
elif age < 65 then "Adult"
else "Pensioner"
let sayHello someValue = #2
let group = #3
if someValue < 10.0 then calculateGroup 15
else calculateGroup 35
"Hello " + group
let result = sayHello 10.5 #4
Experiment with the age comparison: replacing 18 with "test" changes the inferred type of age and exposes errors at dependent call sites. section 3.2.4
Annotations still matter where inference lacks enough information. In particular, member access on classes or C# framework types may need an explicit parameter type; once a type is known, inference can continue downstream. Records and pattern matching often supply enough field or pattern information for the compiler to infer their types without annotations. section 3.2.5 section 5.3.2 section 8.1.5
Functions establish contracts through use
A higher-order function can infer that printer accepts a string and returns unit because it is called with strings. Any supplied function with that signature can then serve as the printer, including a partially applied file-writing function. section 15.2.2
Use IDE type displays such as CodeLens or LineLens to see inferred types as code changes. Prefer meaningful names over annotations solely meant to explain intent, and introduce annotations when they clarify an ambiguity or guide the compiler. section 3.2.2 section 3.2.6
