CODEWITHSUNDEEP
.netf#functional-programmingcircular-referencediscriminated-union
MiP
MiP

Reputation: 6442

Fix immediate cyclic references with discriminated unions

I'm making a mini Pascal parser, this Pascal syntax supports declaring infinite types:

program ::= program identifier ; block .

block ::= variable-declaration-part procedure-declaration-part statement-part

procedure-declaration-part ::= { procedure-declaration ; }

procedure-declaration ::= procedure identifier ; block

I try to create AST domain types for this syntax:

type Program = Identifier * Block
and Block = VariableDeclaration list option * ProcedureDeclaration list option * Statement list
and ProcedureDeclaration = Identifier * Block

But the compiler would throw an error, because I use type aliases, and they are erased at compile-time:

This type definition involves an immediate cyclic reference through an abbreviation

Some said I could fix this problem with wrapped discriminated union types, what should I do?

Upvotes: 2

Views: 129

Answers (1)

Brian
Brian

Reputation: 118865

and ProcedureDeclaration = ProcedureDeclaration of Identifier * Block

Now rather than just a tuple, it's a single-case DU type, which you can create with e.g.

ProcedureDeclaration(someID, someBlock)

and decompose with

match somePD with
| ProcedureDeclaration(i,b) -> ...

Upvotes: 4

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