Any difference between t<'a> and 'a t in F#?

f#

Solution

There is no difference, and yes, they can be used interchangeably even after declaration.

But do note the F# Component Design Guidelines recommendation (Section 4.2):

Consider using the prefix syntax for generics (`Foo<T>`) in preference to postfix syntax (`T Foo`), with four notable exceptions (`list`, `option`, `array`, `ref`).

F# inherits both the postfix ML style of naming generic types, e.g. “`int list`” as well as the prefix .NET style, e.g. “`list<int>`”. You should prefer the .NET style, except for four specific types. For F# lists, use the postfix form: “`int list`” rather than “`list<int>`”. For options, use the postfix form: “`int option`” rather than “`option<int>`”. For arrays, use the syntactic name “`int[]`” rather than either “`int array`” or “`array<int>`”. For `refs`, use “`int ref`” rather than “`ref<int>`” or “`Ref<int>`”. For all other types, use the prefix form: “`HashSet<int>`”, “`Dictionary<string,int>`”, since this conforms to .NET standards

Also, you'll get a compiler warning if you use the ML-style generic parameter list notation, e.g. `('a,'b) t` vs. `t<'a,'b>`.

And while we're at it, note the following recommendation in Section 3.1 of the same guide:

Do use PascalCase for generic parameter names in public APIs, including for F#-facing libraries. In particular, use names like T, U, T1, T2 for arbitrary generic parameters, and when specific names make sense, then for F#-facing libraries use names like Key, Value, Arg (but not e.g. TKey).

(though personally I tend to ignore this recommendation for F#-facing public libraries).

Problem

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